Also makes me wonder if Z-A will do what PLA did and have basically "Pay us extra" missions ala Darkrai and Shaymin, where if you have data for other games on the system, you got quests to get the mythicals.
Presumably they're trying to make older mythicals more accessible so I wonder who they could add.
Right now on modern hardware:
Mew
Jirachi
Manaphy
Shaymin
Darkrai
Keldeo
Arceus
Meloetta
Pecharunt
And technically Meltan/Melmetal are all accessible in infinite methods.
There's also Magearna, kinda. But you need to complete the entire Gen 1-8 dex in HOME.
Well, the Kalos mythicals (Diancie/Hoopa/Volcanion) haven't really been distributed yet. Then we've still got Marshadow, Zeraora, Zarude, Victini, and kind of Magearna, as you've said. I think, going by game, these are some semi-fitting possibilities:
SV: Diancie(it's covered in gems, and terastal energy comes in crystals) Genesect(it's a past pokemon with futuristic technology, tying into the Paradox pokemon)
PLA/BDSP: Genesect(see above but with space-time distortions), Hoopa(makes dimensional portals, which ties in to space and time)
SwSh: Zarude(It's from Galar), Volcanion(a stretch, but Volcanion is all about steam. Steam can be used for power generation, which was what Rose was concerned about.) ,Zeraroa(Same as Volcanion, just with it being an electric type) Victini(SwSh have Pokemon battles as a sport for fame. Leon has a bunch of sponsorships, Marnie is trying to hit the big time for her city and the main protags are literally called Victor and Gloria)
LGPE: Marshadow(An agent of Ho-oh. Since there's no Johto game on switch, the Kanto game will have to do), Zerarora(Lives in a town which has a festival to Lugia. Again, with no Johto game, Kanto is the closest.), Magearna(Turns into a Pokeball. LGPE are all about chucking Pokeballs, moreso than most games.)
Those would be my guesses. They could just repeat some of the mythicals instead lol.