reading this request gave me an idea... could some perk choices determine which of the next choices will be available? kind of creating paths inside the perk tree. Just an idea, if there's 3 choices per level, picking the one in the middle (most versatile but weaker) could allow picking anyone on next level, but picking one in the sides would only allow to pick next from that same side or the one in the middle *only* for the next level choice, as a way to strengthen two defined roles among so many options... or make it only in the one level with the most powerful perks, so to get THAT perk you need to have already picked some other perk in particular.
expanding and combining dubious and lycaninja ideas... could units have more than 2 perk trees each? How nice would it be having one or two common perk tree to all classes, and each class having it's own perk tree as well? ... If there were a way to make that picking a perk from one tree would "cost" xp so other perks re-evaluated wether if they're still available to be selected... so for example there'd be an offensive abilities list/tree, defensive abilities, and class-related special abilities, so each level-up you pick from *one* of the trees. This would mean reworking the way perks become available and make it depends on XP instead of level, I guess.
Well, since Amineri said he's got tons of null bytes to fill I thought I'd toss it here... I'm not meddling with ui or perks by now