I think Mirby meant legendary versions of non-legendary Pokemon.
Which is not happening because a great deal of legendary Pokemon can be outdone by non-Legends anyway, as Regigigas of all 'mans certainly demonstrates (many others suffer limited move pools or oddly balanced stats). A legendary is simply one-of-a-kind, and expected to have high base stats, but may or may not be actually useful.
I don't see a hierarchy between games either. The only reason Kyogre and Groudon [twin slasher] all the others is because they abuse the Ability mechanic new to their generation, and all the oldies got stuck with Pressure. As far as surpassing the folklore surrounding them, well, the legends of the first two generations were more mysterious than anything. Same from the Regi trio. A detailed background for a "legendary" Pokemon was, outside of Mewtwo (which is natural since he was a product of science), pretty much new to 3rd Gen.
Gen4 represented Time, Space, and Deities (once again, with more elements).
You forgot parallel universes (Giratina).
Also, Gen 3 revisited sea and sky after Gen 2 already did them.
Also on DeviantArt, Rumble, DLive.tv, and the Fediverse (@freespeechextremist.com and @bae.st)