For PLA, I'm predicting Shiny Manaphy. Darkrai and Shaymin were available in BDSP (Shaymin had an event in GO, and Darkrai seems to be a yearly Halloween 5-star raid) and Arceus is outright available with just PLA save data. Manaphy has... the egg from Pokemon Ranger. And that's it.
Yeah Manaphy seems like a pretty good bet for either Sinnoh title (or both, if they decide to pair them together -which is how you're meant to get it in L:A to begin with). Especially since the egg from Ranger is like....they let it through, but the way to obtain it as shiny is more through an exploit than intent. Taking the opportunity to "properly" release it after all this time would be nice and finish off those gen 1-4 mythic shinies for real.
As for SWSH? I don't really know, actually. Maybe they could continue with more gen 5 shinies and do Victini or Keldeo, OR they could release Zarude (even if its shiny is very shit). I'm not very sure for now.
Keldeo feels like the safest bet since it, like Meloetta in SV, was the mythic it gave capturable access to.
If they do Let's Go (hard to say the full scope of this, really) I'm guessing they go with the old reliable Mew. If they release Meltan it can't evolve and Melmetal without Meltan feels...off. I mean they released shiny Solgaleo/Lunala without Cosmog/Cosmoem, but...I dunno....just the vibe, you know? I feel like it's gotta be Mew.
BDSP, if they decide to do it on its own and don't want to do Manaphy (or the other gen 4 Mythics I guess), I suppose they could do Shiny Jirachi? It's the other available gift mythic after all. But if there was one title they would go away from an "on brand" give away for, I think it should be this one and it should be a release of Shiny Deoxys.
Deoxys is in such a weird spot. It was only available in BDSP in Gen 8 and never had a give away; strictly speaking you can't fully complete the BDSP National Dex without transfering it from an earlier generation (Celebi had a give away in SWSH so you at least had that). It's in Gen 9 but also hasn't had any releases thus far. Even if you had Go, you can't bring Deoxys into those games without already having a Deoxys in this games.
So the one Gen 8 game that is Deoxys compatable getting the Pokemon that let's it complete its specific national dex in full, would be fitting.