I'm positive Arceus will fight Amaterasu if they actually use it for a DB. That's like the most popular, and it can raise awareness for a game many people find to be a hidden gem (by having Amaterasu get bodied completely in every conceivable way)
However, next Pokemon MU, while it hadn't been talked about as much lately, Greninja VS Espio is still a possibility I can see them doing.
There's also Incineroar VS King (Tekken)
Grovyle has a few potential MUs.
There's obviously Rayquaza, too. Though it's options are a bit controversial lol
And not as likely but still possible are Zygarde, Necrozma (both of them having Cell as an opponent for some reason), Kyogre (Chaos from Sonic and Lapis Lazuli from Steven Universe) and Zacian (VS Sif from Dark Souls)