It's not like the mystery dungeon games already had original bosses before.
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Other spin-offs also had non-Pokemon/original bosses too.
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Honestly my favorite story bosses so far in mainline games are raid-style ones that guarantee capture on defeat just by actually forcing a fight instead of just cheesing with a Master Ball, such as Eternatus, Origin Form Dialga/Palkia, Orgepon, Bloodmoon Ursaluna, and sorta Necrozma in USUM for having a 255 catch rate on Mt. Lanakila.