Does anime!Eusine v.s. anime!Giovanni count? Eusine practically has rabies in that one episode. I half-expected him to start calling Ash a blasphemer since he more-or-less accused him of such. Like seriously, he must have had some extreme anger-management classes or some extreme anti-psychotics in that one special with Raikou and the Gold/Silver player character, because he was a completely different person in that. I'd get a kick out of seeing Giovanni get what's coming to him in a Eusine-shaped package.
Hmm... it'd be interesting to see any combination of the Pokemon Professors do battle. I can only assume that they've all been trainers at some point, so seeing them put their knowledge to the test against one another would surely make for some great battles!
I think it'd be pretty amusing If all regional champions went head-to-head against each other in a tournament of sorts, with the winner being declared as "undisputed champion," maybe (or something along the lines). Personally, I'd want Steven to win, since he's my most favorite. xD" Lance would be a good choice as well, but he's my second option. Aside from that, it'd be nice to see professors do some battling, too.