Who else can possibly be the next top contender at middleweight? Paulo Filho would've been the definitive answer, but they assigned him to WEC when Zuffa absorbed his contract from PRIDE. There's an undercard fight at the Cincinnati show in October between Yushin Okami and Jason MacDonald, who are possibly the third and fourth-ranked middleweights in the UFC right now; however, if Franklin were to win the title back, there'd be a problem having him the winner of the fight so quickly, seeing as he's already beaten both of them in 2007. Evan Tanner and Dean Lister are fighting in December, and both certainly have the credentials to be a top contender, so it may be the winner of that to get the next shot - which means a chance of Franklin-Tanner III. I really don't see any other options from within the UFC currently.
Regarding Cro Cop and a weight drop, that's been a good idea for years now, but as M2 alluded to, he's just not the same fighter he was. He's always fought at 215-220, which is the same weight most light heavyweights rehydrate to after coming in at 205. But I don't think he's losing to the likes of Gabriel Gonzaga and Cheick Kongo because of size differential (taking nothing away from either of those fighters). There's more to it than that, and Mirko has spoken of retirement in the not-too-distant past, and I wouldn't be shocked if he did retire within the next year. What else are you going to do with him? Coming off two losses, who they gonna sell his next fight against? He's signed to the richest free agent contract in MMA history, so it's not like he can be written off to preliminary bouts or anything.