GSP still needs to have at least 3-4 title defences before he gets rushed up to middleweight to challenge Anderson Silva.
If GSP is victorious against BJ, he should have to fight Alves. After that, I would be interested in seeing him fight against Sanchez as well as another fight with Koscheck.
It is a terrible misconception that GSP is a huge welterweight. Granted he is a big welterweight, he would be drawfed by some of the middleweights. Although I believe GSP would have a solid run at middleweight, I think he'll have a lot of problems with fighters like Dan, Rich (yeah, I know he's not in middleweight anymore), Nate (not sure if they would fight since they are in the same camp), Anderson, etc.