I like this idea! It sounds fun, and the temporary slot makes sure
you always can visit it when it appears! It would also make users
more active. All in all a great idea. Full support <3
Support, and support for 7th slot, since I know people who grind
Alola and that takes more than 24 hours (you could miss it
entirely).
As for the method of encountering, I was thinking perhaps do
something like Kyruem, where while interacting there is a small
chance for the island to appear in your hotbar? Definitely a lower
chance then getting Kyruem since this would be a permanent
feature.
Also: Recruitable Wynaut and add a Retro Wynaut that can only be
found here (rare shiny chance)
I support. However since it would be quite rare i believe there
should be like a legendary or something else attached for like a
low chance, although not accurate to the games. Maybe a new ancient
cave quest instead of an occasional thing.