>>49401639>The Lake Trio in gen 4 really should have functioned like this. I don't think anyone would particularly miss them as actual Pokemon.For clarity, the "summons" I was referring to would still be capable of being "caught." (Hell, they'd even still have a PokeBall or whatever.)
HOWEVER, "catching" them would mean earning the right to summon them by 1.) facing some kind of trial and tough battle to earn their respect. Additionally, higher-tier legendaries and mythicals tend to have important "jobs" in the world, so you'd to 2.) 'borrow their time' so they can 'get back to work' purifying the world's water or whatever. Therefore they won't be physically there in your party or PC.
Instead of having them in your main party of six, they'd have their own special ball (Ultra or Master Ball or something) that you could summon them with. Summoning would be almost functionally similar to the current battle setup, just aesthetically different for the most part. If you're familiar with the current anime, compare it to the way that Gou lets Suicune come and go as it pleases.