So I had a thought the other day, they have both Kerrigan and Queen of Blades in the game(technically just evolutions of the same character) so why not combine them. You select Kerrigan as your character and then which version of her you want(Sniper, Queen of Blades, Ascended Kerrigan) works with a bunch of other characters too Tyrande with her bow and Tyrande with the glaives, Illidan as a black mage in war of ancients, for Diablo they could have builds for each of the different trees and in game the different styles/level up trees would change the game play.
although this might work it would cause a couple of issues, first of all new animations would be needed, probably also new models, and your suggestions on ‘variants’ of characters already kind of overlap with others and may be confusing, sniper kerrigan would interfere with nova, tyrande with glaives would interfere with maiev, and to top it off with new abilities you’d probably need new talents.