OK, so, after reading all of the posts in this thread I've come to the conclusion that Warcraft IV would best be suited as a game between The Frozen Throne and World of Warcraft because:
-continuing TFT and erasing WoW is a fan killer (and would probably ensure world war; oh, imagine the anarchy-I mean, we already have terrorism, resource control strategies and unemployment)
-setting the story after WoW would not be a good idea since what happened in Warlords of Draenor (in terms of time travel storyline; too much weird stuff that can create plotholes, fantasies and unanswered questions; alternate universes don't make good stories when suddenly introduced or forced into a story lacking time travel and alternate universe idea for three games and so many expansions) and how so much changed in WoW overall
-making it go many years after WoW is still a bad idea since it could still create plot holes and raise unanswered questions (honestly, better a new BlizzEnt RTS than that idea; sure, so much new content could be added)
-having a Warcraft IV happening between the first and the third games or before the first installment would not make it a Warcraft IV (a continuation) but a Warcraft: <insert specifics here> (would be fun to actually play the stories from the manuals and other stories with developed races like demons, elementals and whatnot)
-turning it into a side story to WoW does not make it Warcraft IV
-modding Warcraft III or StarCraft II or fans creating a game on another engine does not fit as Warcraft IV
-a reboot of the older games certainly is no Warcraft IV!
Now, spinoffs are nice but they tend to go into weird directions. That, however, is another story.
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The gameplay:
-it could just be an advanced version of Warcraft III with more enhancements to hero customization, maybe even unit customization to some extent. Heroes could have spells that could be modified depending on traits and talents chosen by the player either on level up and/or by using some items. A proper inventory for heroes
-they could also make optional quests for multiplayer maps, creep camps with neutral heroes and/or strong creatures that would require a team to defeat them (somewhat early in the game)
-control points would be interesting on some maps, places that need to be held by a team/player in order for them to gain bonuses
-mercenary camps should contain somewhat unique units with different abilities than what the racial/faction units would have. Also, items should not enable abilities a race/faction has
-the reintroduction of oil or a third resource, maybe more?
-ships, at least on/for some ladder maps, you know like in Warcraft II (not all maps were water ready)
-night-day, ground-underground (like in most Heroes of Might & Magic games and adapted to Armies of Exigo); line of sight blockers, natural events
-more than four races would be nice; sure, mixing like they did in Warcraft III is fine; we could have beside the four known factions/races: demon/Burning Legion, naga, can't have too many pandas sorry (MoP came so much later), draenei also came later (WoW:BC), elemental etc. However, some of the factions would need a somewhat sided story (not necessarily flashbacks but things happening nearer to their realm) since for instance naga had a more important role in Cataclysm although being present in most if not all WoW installments (especially in the first one)
-of course, it would be amazing to have a TFT-like editor or at least a version if they decide to overcomplicate it like with SCII's
-LAN would be a luxury
-a custom campaign section like that of the Frozen Throne
Some wrote mixing WoW, WcIII with the MOBA genre. I hope not. It would just lose much to all RTS archetype. This, however, would be more of a spinoff than an actual Warcraft game.
Others implied turning it into a MMORTS. If they'd do that at least let them for a Warcraft game but not Warcraft IV. This would be a more interesting idea than the direction they went with that turned into WoW/MMORGP. Imagine battling armies of players, maps, quests... well, honestly, it's a very difficult task to properly do.
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Story (addressing):
-ingeniously connecting the events of the Frozen Throne with those of the World of Warcraft
-how/why did the factions fuse into two bigger ones; the strife between the factions, why did they make their choices to either join the Alliance or the Horde; why did the Forsaken see their place fir more amongst the Horde (not just because of WoW gameplay geography); why did the belves join the Horde and how did they rebuild Quel'thalas; why did the orcs become enemies with Theramore; how did they rebuild Stormwind?
-why were Gilneas, Kul Tiras, the dragons and other factions absent during RoC
-the strife between the eredar and draenei (this is where a draenei and/or a demon/Burning Legion race/faction could be added to the game) on Argus and the departure of the Exodar
-Arthas' long meditation and why didn't he and the Scourge set out of Northrend to follow an agenda
-Draenor, Illidan and the naga; why was Maiev imprisoned and how she escapes and how "everyone" turns against Illidan
-Sargeras' absence during RoC and TFT
-Pandaria and why/how was it hidden that not even the nelves (maybe from the dragons) knew about it
-introduction of the Val'kyr, Vrykul, nerubians
-why were there people like Thrall's grandmother, Grom's son still living on Draenor and how did orcs and creatures like Rexxar get to Azeroth (when, who opened up portals?)
It would be awesome if
Christopher Vincent Metzen would write the story for this game, if it'll ever be created. I would recommend him doing something like for the first three Warcraft games not like for WoW.
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About modding/using Warcraft III to create something considering Warcraft IV, I can mostly point out some custom made campaigns. But these are more story driven and do not improve/change the gamplay that much. Chronological appearance:
Rise of the Lich King
Rise of the Blood Elves v2.2
Curse of the Forsaken v2.1
The Frozen Tides
Shadows of Hatred v.1.2.4
Others are either alternate stories, do not include RTS, are set before Warcraft III, are spinoffs or are based on an original or different universe.