Scott Brown is likely re-elected in 2012 as no other big name challenger was likely then to enter the field - a Congressperson or two may have entered (Capuano, Lynch or Tierney) and more likely would've been the nominee but I'd wager Brown has the in-built advantage to win. It would've been close but enough for Brown to win.
Now, if Brown is re-elected, does he then run for President in 2016 and/or does he end up as Trump's running mate if he endorses him as a Senator as he did OTL, quite possibly so. Very unlikely he could ever be the presidential nominee in OTL 2016, it's a very slim chance but possible at least and he could definitely run.
As for the Democratic side, there would be a vacuum for a progressive leader from the Senate, so possibly Franken or Sherrod Brown, maybe even Sanders take a more prominent role. Sanders still runs in 2016 but a bonus for Clinton is she doesn't always get the Warren comparison.