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I think based off the title of the games, the new forms are actually Solgaleo and Lunala being powered up with Necrozma, rather than Necrozma being the base
My reasoning is that since it's "Ultra Sun and Ultra Moon" the titles still refer to Solgaleo and Lunala, like Sapphire and Alpha Sapphire are both Kyogre games despite Kyogre having a new form and Rayquaza having much more importance in the remake
If Necrozma was the main mon, the titles would have been more like Emerald, like Star/Prism/Eclipse/etc.
(BW2 I think is an exception since it's a direct sequel, so they had to keep the Black and White naming scheme or else people would have thought it was another third version)
I can see it going either way, but I think regardless, Necrozma is still going to play a huge role, he might be the instigator to power them up or maybe he infects them like a virus (Though Nihilego kinda has that department covered)
