While on the topic, is it just me or would it kind of bother anyone else if the game had the player be the champion for this game? By player, I mean the representation of the past game's avatar.
While it really isn't a self insert for a ton of reasons, the player character is meant to obviously represent the player. Having a stand-in for you wouldn't feel meaningful, but quite cheap and the character would just feel bland and empty, as GameFreak wont be able to implant any form of personality on them.
...And yeah, I realise that Red was literally meant to be a self-insertion of the past players when you battle him in Silver/Gold. But that's a bit of the problem.
As in, when I first found Red and battled him, I had no meaningful recognition or thought. I had never played Red/Blue/Green at the time, so Red was just this blank state with overpowered Pokemon.
When S/M had Red featured as a Battle Tree, he was no longer a representation of the player but literally just another NPC. And a boring one at that. If a new player battled him with the same train of thought I had when I first encountered him, it would be just as meaningless.
However, this doesn't mean that having old characters featured is bad. As long as they aren't there with the sole purpose to have fan service, and play a role in the game as a genuine character, then in my mind they deserve to be there.