Nog & Congo are fairly useless at this point, I feel like we've really seen what they have to offer - and don't get me wrong, Nog is a warrior but he's not big enough to stop the really big guys and can't be controlling enough on the ground (see his fights /w Fedor) to sub the best. Cain & Carwin might perform well, but neither have faced much competition aside from Carwin's win over an overrated Gabe Gonzaga.
I agree about Arlovski, he's sort of like the HW division's answer to Vitor, where you were constantly saying "well if he can get back to being the old Andrei."
Barnett is pretty nasty, although the problem for all these guys (who are outside the UFC) is that none of them are particularly young, and with the limited competition they're seeing now who knows how they'd perform if they were ever to get back on a big stage.