At 145, there's a rematch with Pulver to bring in more money (although, I think a rematch is unnecessary. Pulver did a good job to survive for 5, but he'd be taken out early in a rematch). After that... what? The ceiling on how much 145 can bring in is fairly low. I don't see anyone at 135 who can move up and really challenge him. It's gonna take him moving up to really get challenged and give him the opportunity to pull the cash that his success merits.
I would like that if he go to 155... a great athlete like him could put up the weight without suffer from it no?
Unless, WEC and Zuffa can bring in some top featherweights, especially some of the japanese ones out there, I think he should move in weight to lightweight. There, not only does he have the lightweights in the WEC to take on, but also the possibilty to move to the UFC where the lightweight division is pretty deep. A transition to the UFC would be easy, or at least should be.
The only problem I see, is that Faber is "maybe" 5'6". Pulver isn't a tall dude and he was towering over Faber. At that height, adding weight is going to affect his quickness (his greatest strength, I think).
I know he trains with him anyway, but he should steer clear of any possible matchups with BJ Penn, reason being BJ would put the hurt on him. I can see a fight with maybe Edgar or Tyson, maybe even Joe Daddy or possibly Sherk....although Sherk may be too strong. There could def be some interesting matchups for him in the UFC.
