I will add: My personal priority is the games first (main-line), which is why I've only been discussing pokédex and game-appearances (specially because the Pokédex is accepted to scale across the franchise, which makes everything way easier to scale later)
Which, incidentally, we definitely need a consensus on how to handle the legendary 'mons. Because following my idea on their encounters in-game, it might lead to an abundance of scaling.
Because of the Legendary Birds appearing in Platinum post-game.
My solution to this is to base the scaling depending on the game (legendary birds key for Kanto, another for Sinnoh, and so on), but I'm quite sure many people will be against that.
The Legendary Birds are a problem, because effectively, through them, practically every legendary across generations would scale to late-game sinnoh legendaries. I don't know yet if I could agree to that, so that's why I'm bringing it up.