I think this is kind of a moot point.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.)
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.
Outside of Gholdengo (and even then...), breeding / optimizing pokemon has become *extremely* easy in this generation, to the point you can catch a wild pokemon and make it competitively viable in less than 5 minutes (sources: myself, it's what I do) assuming you are at a game state where you have plenty of resources from previous raids... which you are likely to be by the time the 2nd dlc is released.
Such feature doesn't exactly change gameplay in any real way.
Moreso, obtaining multiple copies of legendaries has been the n1 reason to use pkhex & similar tools for quite long time, and at this point, having a "in-game" pokemon editor would be significantly preferable to "force" every competitive player to use illegal tools on their switch.








