Big list of heroes I would buy probably on release:
Warcraft:
~Lei Shen - a really cool character, probably my favorite character from Mists of Pandaria (bonus points if when he dies he yells in a Mogu voice "useless! all of you!")
~A dreadlord - could be any of them Mal'Ganis/Tichondrius/Varimathras.
~A Pit Lord - really wanted to charge in the middle of the enemy as a giant demon but I don't think we'll see another bruiser warrior anytime soon.
~Voodoo Troll Shadow Priest - Hopefully not as a skin for Nazeebo.
~Grand Apothecary Putress - as an aoe specialist, would love to see a Plague Barrage Heroic.
~Xavius the First Satyr - would look cool and could give him some damage over time curses and stuff.
Starcraft:
~Arcturus Mengsk - heard that there were some files found for him in-game.
~Adept Protoss unit - looks pretty cool
~A Zerg/Protoss hybrid - both units looked really cool. Would not be surprised if they were merged into a single multiclass hero with the ability to adapt/evolve into a warrior (the reaver) or assassin (destroyer)
~A zerg unit that could evolve by altering it's abilities with talents so it could be multiclass and be talented to be a warrior, support or assassin.
Diablo:
Have not played the games, but there is no way you could go wrong with more Angels and Demons.
Overwatch:
I would expect Pharah, Reinhardt and maybe Sombra to be interesting in HOTS but not that keen on any more Overwatch heroes for now.