Since 2006 the middleweight division has been in dire need of top-tier fighters. Anderson Silva has dismantled Chris Leben, Nate Marquardt, Dan Henderson, Travis Lutter, and Rich Franklin on two occasions. No one except Lutter has really tested Silva, and Travis will never get a title shot again after his loss to Rich.
Rich will likely never fight Silva again unless it is a last resort. A possible rematch with Marquardt is likely but would not sell a pay per-view event by itself. So the question remains not who's next, but who's left?
The next person is likely Yushin Okami, who is currently 6-1 in the octagon with his only loss being to former champ Rich Franklin. Yushin also holds a DQ win over Silva in January of 2006. I would predict that Silva would knock Yushin out early in the fight.
After that, the UFC will likely have to go outside of the organization to recruit people like Murilo Rua, Robbie Lawler, Matt Lindland, or Murilo Bustamante. They could try to sign Cung Le or Joey Villasenor but that is highly unlikely.
With the lack of Middleweights out there I see Silva being the reigning UFC champ for a long time.