Which means it won't be Ash.
The time for him to get a Mega has sailed. Especially without Gen 6 remakes.
Oh sure, he could use a Mega in the PWT later, but I'm not comfy with that. Not only does that put him at risk of being somewhat OP, there's little point when Mega-Evolution isn't even a thing in the games anymore...
Though wouldn't it be something if he used Mega-Evolution, Z-Moves, and Gigantamax in one series?
Just not sure if it would be a good something...
The major issue is that Dynamaxxing and Gigantamaxxing can only be done in the Galar region. So there's no reason for it to be main gimmick of this type of anime (world tour) if this took place in Galar I could see your point.
I fully expect Mega Evolution to be reintroduced in hypothetical Diamond and Pearl remakes (come on new Mega Lucario form) and plus it makes more sense than Dynamaxxing which I'm sure will be included but can only be done in one specific post game location.