Virtually every member of the Four has been replaced at some point or other. I think except for Johnny Storm, but I could be off. Reed has been MIA and presumed dead a few times. Sue was side-lined a few times due to pregnancy or Reed just wanting her to be safe, and Thing has branched out for a few reasons.
There is absolutely zero chance of Iceman replacing Human Torch. It makes too much sense and the X-Office are zealots with hording X-characters. It too Ed Brubaker 5-6 years into CA to wrest Beast from their mitts. I've always wanted Spider-Man to join an official roster for real, but I doubt it. Still, he has a better chance than Iceman. The Walrus has a better chance than Iceman. The X-Office would rather they not use a character than another franchise of books use them gloriously. Aside for Wolverine, of course.
Part of me thinks that anyone who is leaving the Four for the moment will be replaced by a character who has shown up in this run before. Hasn't Alex Power/Zero-G popped up once before? Maybe Franklin will finally get a suit and a chance at using his powers without being speed-aged (even if it really does make no sense for him to be under 13 after all these years).