I'd have to say, out of all the characters we know from the lore there are two who interest me most:
1. General Du Couteau, Katarina and Cassiopeia's father. Super-strong dude, blademaster like his daughter and protege, suddenly gone missing allowing Swain to come into power... It'd be interesting to see the abilities this man must have in game and from a lore standpoint, seeing Swain's reaction to his return would be very cool to explore, I think.
2. This one's a bit more obscure, but the man who captured Warwick in one of the Journal of Justice stories. You know, the guy who captured and tortured him, resulting in the Feral Warwick skin we all know and love? I think that sort of angry, revenge-based character would be very cool. (This is part of why I love Urgot. He's a very simple, rage-filled, unwillingly revived hate monster. And it works, imo.) I'd suppose he's someone who was harmed in the Ionian-Noxian War, but with Warwick's lore update, I'm not sure how involved in that conflict our resident wolf man would have been... Still, I'd love to more about that guy!
Edit: Here's a link to that Journal of Justice, btw. The article's called Warwick Escapes Kidnapper.
Also, every thread with IronStylus in it makes me happy. Just sayin'.