So for
my purposes, I never have to care about Mythics. But may as well spit ball
Phione & Manaphy I presume will be a set. I think the research will be based around eggs in particular, and instead of being gifted the Manaphy itself you get the Manaphy
egg that you have to hatch. I can also see, after the release, Phione eggs being added as a super rare encounter in the 10KM eggs. They seem to only do Mythics to tie into stuff so I assume they just wait for some random go fest or tour.
Arceus is probably being tucked away for a "big" thing. Maybe the 10th Anniversary? It's Arceus, after all. I could see it being raid exclusive.
As for its gimmick I see one of two options:
- It's like the Genesects and if you want a different Arceus you have to get the proper typed raid. Means they could also piecemeal Arceus further
- You can change it with some candy and some other "thing" to collect like Mega Energy, Zygarde's Cells or the Solar/Lunar Energy. Maybe it could be fun if it was tied to the typed badges you can earn; you get a plate at each Gold/Platinum and withs ome candy you can then swap your Arceus around. Or maybe you slowly accure type energy as you catch Pokemon. Or maybe you get the appropriate energy out of typed raids (ie: like Necrozma); if they do that hope its just "Mysterious Type Energy" and not 18 individual typed energies for you to choose from.
The Shiny masterwork research will finally require the Sinnoh Platinum medal and will probably be the biggest pain in the ass in the game. To give it more gravitas.
Volcanion is the only Gen 6 Mythic left. Could see them releasing it outside of a Go Fest/Tour and just tie into Z-A's release. Alternatively save it for the Kalos Go Tour since it's not that far away relatively speaking.
Magearna, Zeraora are just full on "whenever." Like Marshadow. I dunno if they'll ever release Original Color Magearna. I'll say again that if they want to "premiere" a shiny in Go outside of the MSG, Magearna's probably the best bet since Lillie's in the anime has been retconned into being the Shiny color.
Pecharunt is probably a million years away, but I bet the lead up to its Go Fest will see the release of the Loyal 3 & Ogerpon.