If he does run, Romney would probably split the establishment with Jeb, which might create better opportunity for one of the more conservative candidates. It'd also make the primary even more of a clowncar than 2012.
It's almost certainly going to be a bigger clowncar than 2012.
Almost certainly running:
Bush
Paul
Christie
Cruz
Perry
Huckabee
Santorum
Jindal
Carson
Fiorina
May run:
Romney
Rubio
Walker
Kasich
Pence
Irrelevant and probably won't run, but one or two might surprise:
Bolton
Ehrlich
Gilmore
Graham
Pataki
P. King