Super Power/Mutation: Kama kama kama chameleon. Lelouch starts manifesting powers or physical traits he observes near him. So if he's around Suzaku, bam, he becomes athletic. If he's near Kitty, maybe he can walk through walls. That sort of thing.
Restrictions: Copying powers would probably result in a much weaker mimicry. Only works on people he's actually physically near.
Reasoning: Lelouch has lived the better part of his life trying to blend in, look nice, smile normally, and otherwise be exactly like everyone else. And even when he's not trying to blend in, he is always tailoring what he says and does to what you might want to hear and see. This is just an extension of that!
Any last questions? D, does that sound reasonable? /sorry to comment with Allen