I’d love to see a fast Fire/Rock type with Levitate or Water Absorb. Magcargo is too marred by low speed to be a huge threat but something fast could be scary, especially with an immunity to one of it’s x4 weaknesses.
I'm going with five.
Isle Abeens: Attract new Pokémon
Isle Aphun: Collect rare items and stones
Isle Evelup: Gain exp and stats
Isle Oamee: Berry farming (gradually unlock more plots as you progress in the game)
Isle Ajoy: Raise happiness (though not as effective as having the Pokémon in your...
Honestly, I could see all of these scenarios being the case, but the one I lean towards the most is an old mythical Pokémon returning with a new form. If Mewtwo (though not technically mythical) could do it with Mega Mewtwo Y, why not Celebi/Jirachi/etc.?