When CoH was being designed, they wanted to make power sets, not character types. When you started your character, you would select your Primary and Secondary power sets. No classes (Blaster, Defender, Corruptor, etc.).
I think that implementing something like this might be nice. Now that there are inherent powers with each class, you'd have to select your class, which would then force you to choose from the existing Primary Power sets. However, you could choose ANY power set for your Secondary. You could come up with some strange power matchups - I'm thinking something like an all pet mastermind like: Demon Summoning/Necromancy. No way to buff your pets, just lots of them!