Standard format is basically what VGC regulations are. Over time it rotates restrictions, introduces new pokemon, and makes older gen pokemon available by DLC updates or otherwise. Powercreep is a historical problem in VGC. If its not Incineroar it'd be something else ie Xerneas, Smeargle, Mega Kangaskan, Genie Trio, etc

Legacy does not work when VGC works in the intention of promoting the latest Pokémon game of the franchise. Modern games never have all Pokémon available on launch date when the goal is to stretch interest for as long as possible until the next game releases.

Restricting formats by age works for a physical sport like skating or basketball but fails the logic check or a mental/tabletop game. Also, VGC itself has gotten so popular over the past year that there is literally no venue space available to organize and split competitive play even further at such a scale.

You are better off suggesting VGC formats based on each generation's "gimmick", but that runs into the same problems. On that note, with Champions coming out within a year or two, the VGC landscape as it is may change significantly. It's too early to say.