I'm not talking about having all the abilities be the same price that would be ridiculous but instead of being a certain rank you spend xp on the abilities. The "better" the ability the more it costs to buy. If you want to upgrade an ability like turret 1 to turret 2 you have to pay more than it cost the first time.
This way the game is "balanced" because you have access to all of the abilities at the same time, just like the weapons you just have to choose which ones to use or in the abilities case buy. The ranks could stay for matchmaking purposes but other than that they don't really have a effect, So you still have to work hard to buy an ability but you don't have to wait until rank 5 allowing you to create more characters faster to what you want them to be. After all Brink isn't about leveling up.
Also if you wanted to get rid of an ability you can sell it for half of what you bought it for.
Just some open ideas, there's always don't fix it if it isn't broken.