How about something like....
Power: If he touches someone, it makes that person want to believe in him.
Weakness: If he does not believe in himself.
Now someone might think that his power is weak..and it kinda is. Like Sanderson said himself, many powers are not useful, and some are pretty much useless. If David gets any powers, I don't expect them to be that useful, at least from the start. Then he will find small ways for him to use it.
As for the other epic that we don't know about, I am maybe thinking that it is Valentine. She has that "get pissed off for no reason" thing that many of the epics we have seen.