McCain already had age issues in 2008 when he was 72. His excuse/defense that year was he was the same age Reagan was in 1984.
In 2012 he'll be 76, so I don't think he'll run.
Rick Perry was doing well until (1) the oops moment and (2) Romney outfoxed him in the debate by running to the right of Perry on immigration (Perry of course being one of the only candidates to be pro-immigration). If Romney isn't there to use immigration as a bludgeon against him, then I can see Perry going further.
If Cornyn, Coburn, and McDonnell didn't run OTL I don't see why they would run here.
One big knock-on is that without Romney, the Kochs won't be as pro-GOP. David Koch in particular really liked Mitt Romney (business background and whatnot), as did Bill Koch (who doesn't get along with Charles but does with David). David also was a big fan of Chris Christie (even more than he liked Romney) and was part of a sizable behind-the-scenes draft-Christie effort. Without Romney, maybe there's more of a push for Christie. Charles meanwhile liked Gary Johnson more than Romney, but went along with backing Romney.
Romney's ability to come back in 2016 would partially depend on his 2008 performance I think. How does he win the primary? There was supposedly a VP deal with Charlie Crist for an endorsement in the Florida primary (Crist ultimately betraying Romney and endorsing McCain) so perhaps Crist sticks with Romney and it's Romney-Crist?
Conservatives won't have a distaste for 2008 Romney the way they did for 2008 McCain (or 2012 Romney). In 2008 Romney was Mr Conservative. I think North Carolina, Indiana, Nebraska's Second, and Florida could go red here. A 311-227 win for Obama wouldn't necessarily be a career-killer for Romney.
I think a lot will depend on what Romney does in the Senate. Will he be a calm deliberative senator or an ardent anti-Obama figure? I can't see him being an arch anti-ACA guy considering the origins of the ACA...