Is it already announced who he will make next millionaire or make current even more rich? There are possible options - first, like said before, it is gonna be either Chandler, or another fight against Poirier (sort of a unfinished business). Second - he needs someone who he will definitely beat and make a loud return to UFC. In other words, he needs a win, someone who he will knockout.
It's got to be Chandler. As a rule, the TUF coaches are matched up against each other. Chandler must've gotten some promise from the UFC that he'll get that fight as he's not pursuing any other fights but just patiently waiting for McGregor. And Chandler's fighting style suits McGregor, he's more likely to win against Chandler than anyone else from the top of lightweight.
Now that Justin Gaethje has knockout Dustin Poirier I think Gaethjhe has already exposed Poirier (...)
I think you're writing Poirier off a little bit too soon. There are some fighters that haven't been the same after a knockout, but Poirier has been KO'ed in a past (by Johnson or McGregor) but managed to bounce back nicely. There's no reason to think it'll be much different after KO from Gaethje (who also got KO'ed and that didn't stop his way to the top).