Emma Willis has a baby due in the summer, which pretty much rules her out I would have thought. She wouldn't be able to film the auditions as she's due around then, and would then have to film bootcamp/ judges houses around the same time as the summer series of Big Brother/ Celebrity Big Brother. I don't see it at all, and they wouldn't let Rylan do it on his own.
I don't think it's entirely outside of the realms of possibility that they'll be so desperate for a 'household name' that we'll get someone from one of the American shows, not necessarily Seacrest himself but someone of that ilk. I don't think that would work at all, but I can see it happening.