Did Malthael even do anything to the demons?
Well, humans were on top as most demons remained scattered or in Hell, true. But most creatures we encounter are either Malthael's crusaders or corrupted angels. Malthael was eliminating anything having demonic essence. I guess it was more off-screen since, we, as humans cared for humans dying not demons so the impact and attention was put on humanity.
Did Malthael even do anything to the demons? I was under the impression that his whole crusade has targeted the mortals, not the Burning Hells. Don't get me wrong, Hell is likely in a pretty bad spot, considering that at the end of D3 all 7 Evils were dead.
Technically, stuff was bad since the LoD ending and instead of continuing with either RoS or DIV they gave us the cheesy DIII retcon.
Still, Diablo 4 is set decades after D3 and seeing how at least Duriel, Andariel and Mephisto appear to be coming back, it can be assumed that the other evils also had sufficient time to be reincarnated.
:\ I won't be able to bear that. It was fine with Diablo, sure because it made sense except for Diablo III of course due to forced story integration. But with the other evils, no thanks. If they pull off something lame like Malthael's death released the demons back into sanctuary where they possessed humans or that Lilith brings them back in a black soulstone or similarly fashion, then they wouldn't have learned anything from DIII's writing.
This would also explain why Rathma summoned Lilith and asked her to "save us".
Nah, I guess, it's about the balance thing that the demons are less than the angels now at least in power and that there's the super nephalem hero out there somewhere. I don't know. Remains to be seen. But again, bringing dead characters back will be lazy and uncreative. They already stabbed us with nostalgia in DIII with Leoric, the Butcher (yeah, yeah, just a demon type not an actual character) and Diablo. And no, there's no such thing as a Diablo game without Diablo argument. Hellfire, Lord of Destruction and Reaper of Souls proved that quite well.
Thus it's totally possible that Hell has been stabilized and became stronger in the years leading up to D4.
Same could be said about the High Heavens. Actually, I'd bet on this last one instead.
Similarly, I think the angels also had enough time to rebuild after Diablo's invasion, so I don't think the Heavens are that weak either. In fact, they should be pretty close in terms of power by now.
Then, what's the point of the save us apart from just stopping the Eternal Conflict which won't ensure balance. It's a fallacy. Humans are the combination of both angels and demons thus infighting is not only possible but valid.
There's a lot of things they have to think through thoroughly so they don't shoot themselves in the foot.
It just dawned at me... If we assume that Heaven and Hell had the time to recover and are now balanced... Well, the Angels have locked themselves up in Heaven, which leaves Sanctuary open to the invasion of the Burning Hells. Perhaps that is the issue that Rathma is responding too? Because Hell taking Sanctuary would likely severely upset the balance. Heck, in the gameplay trailer the guy speaks of some visions that warn him that Hell is coming - perhaps it's literally happening?
I think Hell will be coming with Lilith. It would make more sense for me. Otherwise, Lilith would have no trouble ensuring a nephalem empire under self rule unless, again there are other nephalem to say something against that, the most important of them being the RoS hero. Also, I don't think the Horadrim, few as they are, will be too happy about a demon ruling Sanctuary. Tyrael, surely wouldn't, not because he's 50% human but because the character realizes neither demons, nor angels helped humanity but used humanity for various purposes or even for none at all, only hatred being cast upon them.
Once again - I don't remember Malthael doing anything to the demons and outside of their invading forces that Asmodan, Belial and Diablo brought with them, the Nephalem also didn't descent into Hell to "decimate" demons. They've surely taken pretty heavy losses, but I don't think most of them were "annihilated". Especially considering that according to modern Diablo lore, Demons automatically reincarnate after some time anyway.
Well, the eternal "reincarnation" thing was also said about the angels through their Crystal Arch. Now, this could happen but if it does, we'll have a reboot game and I wouldn't like it. With Diablo III, at least they tried not to do it in our faces but behind the curtain with the black soulstone.
I don't remember exactly. Perhaps Malthael wanted humanity gone so that the Eternal Conflict continue as so long before? I have to check on that.
At any rate, if Rathma wanted to make demons great again, he wouldn't have summoned Lilith - she's very anti-demon and chances are she would be more interested in finishing the job than rebuilding Hell. Obviously, it's uncertain - perhaps she'd want to take over and that would strengthen Hell, but then again - in order for Lilith to get control of Hell, she'd need to fight it, which would temporarily make it even weaker. And as I've said, there's also a risk that she'd want Hell dead anyway.
Yeah, we don't know exactly what this balance of order and chaos means. They have not explained it. Does it mean an Eternal Conflict? Lilith wouldn't want that. Maybe Rathma will use Lilith? I doubt it? Who the heck knows. The character needs Lilith for some reason, for sure since I don't think it's just family affairs when Rathma and Lilith both have objective goals for which they'd fight for. See Inarius vs Lilith. Inarius vs Rathma in external material.
Honestly, I'm at a loss since part of things make sense, part don't. It's like 50% angel, 50% demon->humans->indecisive nonsense

Maybe you can figure something out as we brainstorm.
Though, during their panels they've said that after Malthael murdered half of Sanctuary, there was a huge power vacuum that was filled with religion. Now, if the Nephalem wanted to subjugate the world, there wouldn't really be any power vacuums - he'd be in power.
Hmm, were they specific that it was so before Lilith was called back? In any case, an empire is an easy thing to make. I mean, sure, the nephalem is super strong but it's one man. Rebels, revolts can happen, maybe even Tyrael and the Horadrim being part of a resistance?
I also don't think that they'd want to imply that our D3 character turned evil. Some people wouldn't like that.
Who cares, lol? This is Diablo not correctness politics.
Remember the first Diablo? As one of the three said in the first Diablo IV panel. Diablo didn't have a happy ending. Albrecht wasn't saved and the hero was corrupted by Diablo. Also, Lord of Destruction was practically the end of humanity until they retconned it for DIII.
I feel they will want to distance D4's story from D3 as much as they can.
Honestly, with all the cheesiness in DIII, I don't want them to ignore the whole thing, just the cheese. RoS was pretty good compared to normal DIII, overall I mean in terms of story. DIII definitely had its good parts.
Like heck, they can say that the Nephalem died of old age or took the other emerging Nephalem with him and simply left, having came to the conclusion that people suck.
Lol, come on. The nephalem was killed while sleeping, poisoned. They already did a McGuffin with humanity forgetting the past after Lilith and Inarius were stopped and the (or an) armistice between demons and angels was agreed. Only Rathma and Kalan, the first's disciple remembered and they disappeared

Boy, I sure don't like characters disappearing and reappearing later out of the blue.
Remember that as powerful as Lilith is, she's only a daughter of a Prime Evil and at the time Diablo destroyed the Diamond Gates he was a Prime Evil, i.e. an incarnation of all Prime and Lesser Evils combined. That's a massive power difference. Plus Angels have likely installed some extra safety measures after D3, so... I highly doubt that Lilith would be able to break into Heaven even if she wanted to. Thus, perhaps she needs Inarius for some back entrance tips or whatever.
Sure, but Lilith will have a nephalem army, or will corrupt the nephalem hero or Rathma and some demons and humans will be enough. We'll have to see how safe the High Heavens are. Also, Inarius, you think the character will help Lilith after what torment has been brought upon? To help, the angel would have to be free to some extent so torture won't work anymore especially after so many years of tachyphylaxis.
She did. I think it was stated somewhere in the Sin War books that while Inarius wanted to get away from the war and have a safe haven, Lilith considered the whole Eternal Conflict stupid and wanted to end it - and once she noticed how powerful the Nephalem were, she wanted to use them to do so. And I don't know about you, but seeing how angels and demons hate each other, I doubt that there was another way to end the Eternal Conflict than to wipe out both groups. I mean, any subjugation or whatever just wouldn't last.
OK. Let's keep that in mind. Then, would nephalem be better and not start another conflict since they are made of two opposing resonances? If they are fitted together, it doesn't mean they are peaceful. That is proven in humanity's character and no it was not because of demon or angel influence. That would mean angels actually cared about Sanctuary in a way or another.
By the way, there was that thing with the prophet and the Vizjerei. Eirena, the mage companion coming from the past, the prophet being an angel who sent the companion to the future for a purpose. So, now angels see the future!? Also, Inarius was the prophet of the Cathedral of Light. Maybe, it's the same angel or one of those who worked with Inarius back then. But what happened after? Where did the Light come from to the Zakarum? Diablo II says the Light comes from the High Heavens. So, how does humanity draw it from there if not with the willingness of the angels?
External material says the Light is found within humanity and can be accessed through teachings of Akarat and that the Light was brought by an angel (Prometheus

) to Sanctuary.
Weirdly enough, you can find that info here
The Arreat Summit - Monsters: Zakarum Zealot for Diablo II. No idea why this wasn't put in the manual. However, this source does not say anything about the Light being brought down or inner Light.
Why would subjugation last on the nephalem? What about demons and angels somehow will be totally eliminated (no reincarnation/revival) and then the nephalem will turn against Lilith and the franchise will end with only nephalem existing?
I very much doubt that Inarius is just sitting there without any defenses... Like, come on - he's tortured by Mephisto, who apparently is back, so that's one major obstacle.
Zarathustra Moses Jesus Muhammad. I hope not. Mephisto was killed in DII long, long after Inarius was already imprisoned. Now, somehow, the villain's cliche return will make the demon nostalgic about torturing Inarius?
The only defense Inarius could have is being hidden. Apart from that, the angel is battered, weak, deformed and unimportant to any other demon or angel. No one cared to use the angel for anything than torture. Actually it is mentioned the angel was left there alone for eternity to see its own deformities as the place was basically a room made of mirrors and the character had no more eyelids, as if angels have any face, lol.
Heavily guarded? Biggest prize? No. Inarius was first considered a mere nuisance and afterwards left to be self tortured. That's what the sources say. Also, external sources point that Mephisto took Inarius to Hell so, not sure if that's the villain's dominion as in the Act III of Diablo II which wasn't. Interestingly this source
Inarius says Inarius was denied death. So much for eternal revival, eh?
Perhaps something happened that caused Nephalem powers to wane? It's been a long time, so perhaps Nephalem blood is so diluted/weakened now that there's just not enough Nephalem to fight Hell? Or maybe they're just too weak? Or maybe RoS hero took all Nephalem with him somewhere? Who the hell knows.
Sure. Lilith and Inarius are also weak, especially the later. Also, who would want rabid nephalem people? Imagine that Lilith is the Daughter of Hatred and Inarius will be pretty much full of hatred and insane after so long there alone. Do you think the angel somehow succeeded in becoming blind and meditated all this time

?
Because it's a suggestions thread and we're talking lore
Sure but it's about DIV in general, suggestions/comments pertaining all the aspects even the story. If you like, you can open up a new thread and we'll ask higher staff personnel to move the posts or we'll just link them there in one post.
Also, who's interested for more Diablo IV stuff:
Diablo IV Feature Overview
Diablo IV Unveiled
Diablo 4 - A Return to Darkness BlizzCon Panel Recap
BlizzCon 2019 Artists at Work: Diablo
All the Diablo Announcements at BlizzCon 2019 - Diablo IV, Lilith Wings, Diablo Immortal
BlizzCon 2019: Liveblog Diablo IV: Systems & Features Panel
Diablo IV Talent Trees and Skills
The Art of Diablo Recap
You have to scroll down on those pages to get to the content.
Also:
Diablo IV Feature Overview