In my opinion, without question, Khabib vs. Tony and Conor vs. Nate are the 2 fights to make. Those are 2 GREAT, headlining fights that MMA fans have been wanting to see for years. Conor always does big numbers and with Khabib becoming a bigger star after dominating Conor (and as much as people don't want to admit, jumping over the cage), I think Khabib vs. Tony does pretty well, too. No matter who wins, it sets up another great title fight afterwards. Shit, you could even have the losers fight each other and it would be a big fight.
The problem is, if they make Khabib-Conor 2, I'm still going to watch it. And so are probably 2.4M+ other people. And the UFC knows that. Tony would be still be there after, so why not strike while the iron is hot?