Although looking to 2016 this early is meaningless, my list would be as follows:
Kirk, Toomey, Ron Johnson (these three are obvious)
Portman, especially if Tim Ryan runs, but either way Portman has embarrassingly low name-rec for a Senator, only 42% of Ohioans knew enough about him to give an approve-disapprove opinion in the last poll I saw on the subject.
Blunt (especially if Nixon runs, but even if he doesn't, Blunt has never struck me as a super-strong candidate and it's not like we don't have a bench here)
Iowa (if Grassley doesn't run for reelection)
Rand Paul (if and only if we can recruit the right type of candidate and Paul wins his primary)
Rubio (if the FL Democratic party can get its act together and find a decent candidate)
Definitely Burr (no one seems to like him for some reason
)
Ayotte (but only if it looks like a neutral or Democratic-leaning year nationally).
