Basically, that when he "returns", that he offscreen caught, or obtained a new Pokemon that isn't just his ace, but that it also feels like a very high wall to climb over...
Myself, I would have given him Raikou.
First of all, legendary Pokemon are always portrayed as powerful, and second we are at the point where even the likes of Dragonite aren't all that "special" anymore...
Other than that, in anime, Raikou always felt kinda underused, and overall just looks fitting for a legendary Pokemon trainer.
Raikou being electric type, which is rare for Ash's Pokemon would also be interesting to see.
...
Of course, none of that will ever happen.
Myself, I would have given him Raikou.
First of all, legendary Pokemon are always portrayed as powerful, and second we are at the point where even the likes of Dragonite aren't all that "special" anymore...
Other than that, in anime, Raikou always felt kinda underused, and overall just looks fitting for a legendary Pokemon trainer.
Raikou being electric type, which is rare for Ash's Pokemon would also be interesting to see.
...
Of course, none of that will ever happen.