>>34432536USUM's story was missing a real villain. There could have been a captain of the Ultra Recon Squad that was secretly playing Lusamine and the URS' motivations against each other to control Necrozma or something. Lusamine could have kept her obsession with the Ultra Beasts, with this new villain promising to give her access to Ultra Space if she in turn develops a way to restrain Necrozma, ostensibly to prevent it from stealing the light of other realms. Then, after defeating Mother Beast Lusamine, this new villain could use the equipment to drag Necrozma out of Ultra Space to combine with Nebby, just when everyone thought things were getting under control.
The thing is, it's such a simple and obvious solution that I'd have to assume that Game Freak deliberately decided NOT to include a villain like this because they were worried that it would either be too similar to other villain plans in the series, or it would take focus away from the fan-favorite characters. Neither explanation excuses introducing all the messiness that USUM ended up doing.
