GSP mostly fought at Welterweight, and also beat Bisping for the Middleweight title, so he fought in a higher weight class than Khabib. However, there were rumours fairly recently that the 2 were going to come out of retirement to fight each other, but that was pretty much squashed. I'd imagine Khabib would have fought up in weight since he's definitely not walking around as a LW nowadays. It would have been fascinating to see, though GSP is 40 now while Khabib is still technically in his athletic prime as far as fighters go (although Glover Texeira just won the Light Heavyweight title at 40 so who knows!)
Paddy is an interesting one. He definitely has that star quality, and he's an exciting fighter to watch for sure, but he was nearly taken out in his first 2 fights in the UFC. That being said he did recover well and turn them around. It's early days, and if he learns to protect his chin a bit better (better guys would have finished him), then I think that he'll be a problem for a lot of guys in that division.