I'm still thinking the list will be pretty darn restricted. I think Game Freak would rather cut us off hard-to-get-in-game but non-legendary and non-mythical mons like starters (especially past-gen starters - imagine us asking for Hisuian starters) or Gholdengo. (Game Freak even went to the length of cutting us off farming Gimmighoul Coins by baiting wild Dittos into profitably Transforming into our Gimmighoul-line Pokemon - Gholdengo's sole ability makes Transform fail (to the point that I think Game Freak built Good as Gold around this interaction), and Dittos Transformed into Gimmighoul drop no Gimmighoul Coins when defeated - so I don't think they'll even let that pass through.)It doesn't exactly have to be that complicate, since Mythicals have some "list" in the code, so if they really wanted a "in-game-pkhex" they can just have it exclude mythicals and/or legendaries in the same way VGC filters them out.
The only legendary(/mythical?) I think Game Freak will make an exception for with the "sanctioned (one-shot?) PkHex" is Terapagos. It looks default Normal-type (look at the raised Normal logo at the top of its shell!), has (probably) all the other type symbols on it, and therefore looks insanely customizable. It's also a Gen 9 mon, unlike its legacy counterparts Arceus and Silvally, so us getting one (or multiples?) right now won't be so bad.