Eh, I keep hearing about how Horford is a natural PF, but I've yet to see him look it in games. Dude seems to be an undersized center. I don't think he'd fit with Asik or Jordan, for example. He could make it work with Splitter, but I think that would be a step in the wrong direction for the team.
I wouldn't mind Al Jefferson. I know people complain about his D, but he's one of the top offensive players in the game, in my opinion. I think these last couple of years in Charlotte have really helped develop his game on both ends. He's not a good defender, but he'd be decent with Splitter.