Notes:

In all seriousness, Andross, Majora's Mask, Mother Brain and any Kirby final boss, all of which are currently assist trophies, would make fantastic additions as fightable bosses, and all of them have different forms or similar counterparts than their assist incarnations that could feasibly be used (to say nothing of the fact that Rathalos already sets a precedent for the exact same character appearing in both roles).

And as much as I love Giygas, I'm just not sure implementing him in a physically fightable form would even be possible without losing the unique intangible terror that made him (in)famous.