True, although all things considered I don't think that's a huge thing to ask for; lots of games have character customisation far more complex and detailed than picking between two or three models that are practically identical save for a couple of very small aesthetic differences. Even if they DID include every character from past games as a playable avatar, it wouldn't be particularly standout in the grand scheme of things...if anything it'd still be restrictive, since you wouldn't be able to change height, skin tone, hair style and colour, etc etc. These are all standard features for games that give you a self-insert protagonist...except for Pokemon, anyway.
I think they're far more likely to opt for the clothing customisation route as well if they're going to do anything with it though, as it was a pretty popular feature in the Kalos games and it's a much easier way to give players freedom of choice...and it allows for greater flexibility too, as you'd be able to change costumes at will, plus there are all the other potential costumes they could stack on top of that...dressing up as an Elite Four/Champion character, for example, or even characters from other franchise...you just KNOW they'd have Link and Zelda, Mario and Zelda, Splatoon, Fire Emblem and Metroid costumes. Maybe it's not as much of an homage to earlier titles, but it IS a nice little nod and a cool feature nonetheless. We can dream, I suppose. xD
...just as long as we don't have to play the games using Miis. If that ever happens I'm swearing off the franchise.