I know a valid reason some give that GW's maps have balance issues, which I largely agree with, is that the rooftops dominate the action which means Foot Soliders and Peashooters, which are already the best general purpose characters in the game, rule the battlefield. This is a serious problem and one of my very few complaints about the original game.
I'd be quite happy with giving every character some kind of option to get to the high ground, whether through map design, or abilities, or a combo of both. Just make them fit the character and vary things up a bit so they don't feel too much alike.