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I'm not sure how well this could be applied to most Pokémon, but I had a concept for an in-battle spin on fusion (rather than an out-of-battle form change like we have now) for the purposes of Necrozma before USUM, to distinguish it from Kyurem.
The idea was that there'd be an item based on the triangular shape on Dusk Mane and Dawn Wings (what we now know is Necrozma's Brain Prism). Rather than fusing directly into Solgaleo or Lunala, Necrozma has to be taken into the item itself - as with Reshiram or Zekrom, this makes it go away, but you can set it free at any point.
Then you'd give the Brain Prism to Solgaleo or Lunala as a hold item. In battle, Solgaleo or Lunala would have a fusion trigger button on the lower screen like for Mega Evolution or Z-Moves; if you press this, you see Necrozma take over control, and in effect, your Solgaleo/Lunala is gone from the battle, replaced outright by the Necrozma you put into the Brain Prism, which is now Dusk Mane or Dawn Wings.
Since it starts at a clean slate rather than carrying over anything from the fused Solgaleo or Lunala, this can effectively be a seventh Pokémon, but only if you use it correctly (activating it too soon means you don't get to use Solgaleo/Lunala, but activating it too late risks Solgaleo/Lunala fainting and missing your chance).
I dunno; I can see this being a little overcomplicated, but I expect it would be pretty easy to get the hang of it once you actually see it in-game, and it would add a unique flair to how the fusions are used in battle. I guess that wouldn't really work with Ultra Necrozma, but it might be a cool thought for any future fusions, particularly if they're possession- or parasite-based like Necrozma rather than something more likely to stick together for long.
