I think bringing in the brother is way too obvious, and it doesn't really sit right with the movie. They didn't explore Cyclops' life at all while he was alive, so why would they want to after he's been shoved off the mortal coil?
Also, why on earth would a new mutant who hasn't been part of the team come in and suddenly be the leader of it? That makes zero sense at all, no matter which mutant it is. Sure they could get some easy angst of everyone being unhappy being bossed around by the new guy, but that's just it. Easy does not mean good.
What they could do is bring someone in to the team that wants to be the leader and consistently hampers the next leader by trying to usurp them. Probably unsuccessfully but I can see it causing a major setback to the team because half listen to the real leader and half miss the announcement and the person trying to become the leader tells them something else. But what mutant character would fill that role and want to have power to boss the Xmen around?
Honestly I think Storm's the most responsible character and thus obviously would step up as leader. She's proven herself good at tactics in the movie universe (she's incredible with them in the comics but that's rather irrelevant to the movies at this point) along with being the most powerful mutant on the team now (well, she should be; the movies don't exactly support that).
What would be really neat is if they brought in another mutant villain that requires the brotherhood and the Xmen to join in to fight against. Sinister or Apocalypse. And you could bring Cable in..