Some of them certainly have what is needed to return in later generations. Rayquaza is one of those since it lives in the ozone layer making it possible for it to travel to other regions. Celebi, while a mythical, is another one that has reason to be in other regions.
Other legendaries not so much, unless you can come up with something creative for a reason it's in the new region. Like, Mewtwo was created in Kanto, so why would it suddenly appear in a random cave in Kalos?
Not much of a fan of how legendaries just randomly appear in recent games. Though, I do like how SM handles this issue, by simply not doing it and just mainly sticking with their regional Pokedex letting everything else be handled by outside resources.
As for new legendaries: it's definitely necessary to introduce new legendaries, just so you have something to work with that represents the mythology the region is built upon. In that sense, different regions having different amounts of legendaries makes sense, because even irl you have religions with different amounts of deities; for example, you have Christianity which only believes in one god and say Roman mythology who has tons of them all doing their own thing.