I'll skip my Cruiserweight rant here, and restrict myself to the existing weight classes... Anderson Silva up to Light-Heavyweight.
Not much to add here, except that I would actually prefer to see Silva move up than fight GSP. Call me a heretic, if you want. Rich Franklin back to Middleweight.
I think Silva is the only Middleweight Franklin probably can't beat, and the division has changed a lot since he was El Hombre. He's never fought Sonnen, Marquardt, Maia, Bisping, Akiyama, Belcher, Stann, Leben or Palhares. Tyson Griffin to Featherweight.
He's lost 3 in a row, and 4-out-of-6 going back 2½ years. He beat Urijah Faber 5 years ago, but I don't know what weight-class that fight was in. Pat Barry and Jon Madsen to Light-Heavyweight.
Neither guy is a giant, and I have no idea whether they could in fact make the cut to 205, but I think they should look into it. I don't think either one of them projects as a Top-10 fighter in the UFC (in any weight class), but if they could make 205, I think they'd both be title contenders in Strikeforce's Light-Heavyweight division.