Dose anyone really think that Mitt and Rick can kiss and make up?
Reagan and Bush were despite the latter's "voodoo economics" bit, so kiss and make up isn't what might stand in the way of this.
The last thing Romney needs is a Republican governor as a running mate, as the inevitable comparisons to what Romney did and didn't do as a governor is not something that benefits Romney, so that disqualifies Christie and McDonnell.
If he is the nominee Romney needs to pick someone with Congressional experience. Rubio doesn't have enough Congressional experience yet. Santorum has the needed experience, and if Romney decides he needs someone who has also run this time, he is the best of the other Republicans who ran to be Romney's running mate. However, Santorum will be controversial and he has been gone from Washington these past six years. Romney may choose to go with a sitting Senator.
There are seven current Senators that I think should be on Romney's long list.
Richard Burr (NC)
John Cornyn (TX)
Mike Enzi (WY)
Jon Kyl (AZ)
Jerry Moran (KS)
Jeff Sessions (AL)
John Thune (SD)
They each have their strong and weak points, but they all have a reasonable amount of Congressional experience, no time as a Governor, would not put a seat at risk for a Democratic pickup, and are not so controversial as to be toxic to either the base or the middle. If Romney picks a sitting Senator as his running mate, it should be one of these seven.