there has to be a line where things arent hopium and just delusional
I think the only part where it's not just hopium overdose is expecting whatever is in ZA to be relevant *in gen 10*.
It's very likely that at least part of whatever new comes with it will be available in gen 10, most likely after a bit once Home compatibility happens.
What.... eh, rough question.
New pokemon / forms?
Sure, almost guaranteed. Some may have slight stat changes, but very likely to happen.
New moves?
Maybe. Some of the LA moves made it to the main games, albeith many got reworked to match the mainline combat system
Mechanics?
Eeeeh. #doubt. Legends games are at core pure single player experiences and many mechanics get altered for the purpose of those games and only limited to those games. I've said many times for people to not expect Frostbite to make it to main series and people drowned in copium about it for half a year.
Supermechanics?
Hard no. This is where the line draws into copium. Styles were the "supermechanic" of LA, and it was clearly designed as a strict single player game mechanic due to the nature of the combat system.
People think that Megas will reappear, and I think that's likely, but I am quite confident that similarly to Let's Go they will stay there, even if we were to get new megas. Main reason is that it'd overlap with whatever a gen 10 supermechanic is.
We already seen in gen 7 how confusing having multiple supermechanics can be, and I am quite sure National Dex here on smogon is another clear indication of how throwing toghether multiple supermechanics is an absolute shitshow of balance since you have to account for several oneshot-buttons (don't forget, VGC doesn't do bans).
The only scenario in which I would see ZA introduce new Megas **AND** them reappearing in gen 10 games is if the Mega evolutions
are the gen 10 supermechanic.
Which bear in mind, I don't consider impossible, I honestly am not sure what else they can come up with after we had, in sequence
- Form changing transformation
- Actual oneshot button
- I become godzilla (also oneshot button)
- Wanna see a magic trick? I'll make the type chart disappear
Them moving to re-use older games supermechanics can be a thing (expecially for something like Megas that feed into nostalgia). But I wouldn't exactly bet on it and I think it dangerously dangles on the "lethal dose of copium" line.