I think there are a lot of great middleweights out there for him to take on. People seem to relate a lot to the Ultimate Fighter since it came on the scene, but most of these guys aren't ready for the type of competition Anderson Silva brings to the table just yet. I think guys like Jeremy Horn, Matt Lindland, and Nathan Marquardt would be a good solid test for him right now. Especially Horn, who has never been K.O'ed (not counting his "T.K.O" to Chuck Liddell which I still see as a verbal tapout) in his entire mma career, and has only been knocked down by a strike in a fight by Chuck. Although Anderson has already beaten him once by a unanimous decision, it would really show the mainstream mma fans that not only can Jeremy Horn take a WICKED punch, but also that Anderson is just human, even tho he looked like a machine against the fan favorite Leben.
On another end of the spectrum, I would like to see him go up against Evan Tanner, the former middleweight champion, as sort of a proving grounds to get to Rich Franklin. Nothing gives you more credibility in the eyes of the champ than convincingly beating the guy he took the belt from. It would be a quick and easy way for Silva to get into Franklin's head. That's my 2 cents.