I agree with you about Overeem, he's a great fighter, but I don't think it's likely that he'll ever be a champ (esp. in a competitive a huge org like PRIDE). I think either he needs to stop fighting so damned often, or he needs to take a break and just train, train train. When he's on though, he can be pretty dangerous.
Nakamura is solid. He just has to find ways to finish his opponents, and he needs to work on his striking. It'd also help if he worked on his submission skills, but I think in a few years he should be near the top.
Antonio Rogerio Nogueira, or Minotoro, can be a champion in this division, or at least it's strongest contender, when Wanderlei hangs up the gloves and passes it on to Mauricio Shogun, who I assume will take it. His fight against Shogun was a war, and I think when Wanderlei leaves, him and Shogun will be the top 2 in the division.
favorite MMA fighters
A. Rodrigo Nogueira
Mauricio "Shogun" Rua
Georges St. Pierre