Some random thoughts:
1) Make some skills auto cast. In the beginning you need strategy, but when you have plenty of mana you just spam a lot of skills. The siren ability is a perfect example. 2 sec cooldown and I never run out of mana. Still I need to click it again and again...
2) Revive the grunt. Sometimes he just dies and it's annoying to progress in certain parts of the map without him. If the grunt dies, revive him. Call him " Injured Grunt". Make him useless in combat ( so you still penalize those who are not carefull)
Now that you can have a bigger map there are plenty of things you can add if needed without worrying so much
3) Add more masteries to the heroes. I like ranger but I don't care about pets, so the mastery stat is useless to me. If you could have 2-3 masteries (only 1 active) that problem would be solved
4)Each hero has a special item only he can get. Take this a step further. Something like an "artifact" for each hero. Make this upgradable in multiple ways so each person can choose the right one. This could be a really good gold sink. 3000 gold to buy, 4000 to lvl 2 and 5000 to lvl 3. That's a lot of gold and it won't be easy to get. It's very risky but it will reward good players.
5)STR/AGI/INT only shops items. Same theme. Make a shop that sells items only for certain groups of heroes or certain heroes. SO only AGI items, or only Hunter items.
There are very few items that give you stronger skills or are "tailor made" for a specific hero. This would give the same feeling as talents but with more investment, so you feel better when you finally get it.
6)Add a vault near the world tree. You could have a command on a backpack "sends S1 (first slot) item to the vault", or from the vault to a player. This would make the game a little faster and help organize all the items ready for upgrades. Add a worker( that can't leave the area) and it's even better. The idea is to make the game more fluid, so you don't spend too much time wondering about, you can spend it fighting. This would reduce game lenght by 10-30 minutes.
7)add a single player -test mode. at the very least give the chance to -repick or hire more heroes. I wanted to check out the map again and I spend waaay to much just restarting the game and looking at heroes(to find one I like). You could add infinite money,max level and so on, but you made it clear it's not something you want. The repicking however ( only in single player mode ) would be esentially the same as restarting the game, so no harm done.
8)Write on lesser items what you can used them for. The reverse of recipes
9)Last but not least. You said many times that you don't want to organize the item list. If I remember correctly you don't want to " force " people into buying certain items for certain heroes. You can keep every shop the same, only add one shop(or maybe more) that showcases the items. Maybe it gives you the chance to buy them at full price without clicking. For example, I wanted to buy a mastery item, I spend way to much time searching for them, and them I spend time finding out what's the best. An example:
Shop-
1)ON-hit effects
2)Orb-effects
3)On-kill effects
4)Getting hit effects
5)Life steal
6)Armor/reduce damage taken
7)Auras
8)Buffed summon
9)Mastery items
Etc.
No problem if some items go into multiple lists. This would make it much easier to find the item you need, and also let you chose whatever you want without feeling constrained...
10)add an online walk trough. Just played with a friend, had a lot of fun, but couldn't get past something. We ran around the map for 20 minutes but he got frustrated and we left the game. That is one player that won't be coming back sadly... I get that it's a part of the experience, but W3 has a really small community. If this where a big and popular game you could find all walk troughs,secrets etc online in a few seconds, so it wouldn't become frustrating.
Hope some of this helps. As always, the map is better with each version...and as always I had a lot of fun playing it!
EDIT:
Now my ideas are all over the place, I know... Some might be good, some might be not so good...but this one I am 100% sure it's a good one and fun

. I am playing the tide-runner and I have a lot of DMG reduction. Give training dummies (or just one specic training dummy, maybe in the "base" ) a skill that deals 100 normal dmg and another skill that deals 100 magic damage in a very close AOE. You could just make it an immolation aura so it's easy, call it " burning training dummy". This way you know how much DMG your hero takes.