Have Romney win New Hampshire, Florida, California, and Georgia and I think McCain will drop out after Super Tuesday.
Romney won't have as many issues with the base, so he won't have to pick someone like Palin to shore up the base. However, a lot of folks aren't gonna be that excited about a Mormon candidate (something that hurt Romney in 2012, sadly).
Still, his financial background likely means he'd be easier pickings for Obama or Clinton given the economic crisis. This is the man who is on record saying that he likes to fire people (yes there's context but context isn't going to be in headlines).
2008 Hillary was much stronger than 2016 Hillary. She didn't come off as being as socially liberal and was more moderate on hot-button cultural issues like guns and abortion - likely due to having been a fairly responsive Senator for upstate NY. Her turnout among young people and black voters would be weaker however.
According to game change, Hillary Clinton's top two picks for VP were Ted Strickland and Tom Vilsack.
If it's Clinton instead of Obama:
Clinton wins Montana, Missouri, West Virginia, and perhaps Arkansas and Kentucky. The interior south swinging against Obama just wouldn't happen with Clinton. Part of the hate of Obama in Kentucky was due to his denying the Clinton restoration (not that they're too fond of Hillary 10 years after the election). I include Kentucky because even today the Democrats have a registration advantage in the state.
Clinton loses North Carolina. It was a paper thin win in 2008 historically and HRC wouldn't have the same youth and black turnout.