I guess Stars should be Necrozma game but knowing GF and Nintendo they'll try to maximize profits by releasing two games, possibly sequels

What forms Necrozma could get though?

Maybe Necrozma + Zygarde fusion as one form and Necrozma + Guzzlord fusion as second?

Why these two? Well Guzzlord and Complete Zygarde do have a clear similarities (both dragons, with huge HP as their biggest stat, similar design aspects, Terminus cave and Resolution cave etc) and not to mention Zygarde being in Alola is stil a mystery and Guzzlord also have its own secrets (it appeared in Alola in the past unlike most other UBs)

Necrozma does have some dragon traits in its weird design so that makes it likely to have some similarities with the other two.

It came from another world but stayed here for a long time so it can be interesting what it really is: a friend of this world or a foe. By fusing with Zygarde it could be the ultimate protector form and with Guzzlord the ultimate invader form