CatRaccoonBL
You can do it!
I was with you until you said "blackhole." The problem with blackholes and Rosalina is that the blackhole is an opposing force. Something that stops Mario on his mission. If anything, I would think Rosalina would want nothing to do with blackholes.The Comet Observatory could have been used just like it was in Galaxy, and it could be implemented through a mini-cutscene like Mega Man and Captain Falcon's Final Smash. Maybe have Rosalina conjure up a black hole from Galaxy, and if it hits, it activates the move.
They even have an actual cutscene to use as reference in this case.
Though, the cutscene thing does sound like a good idea, just not with a blackhole. Plus, that would make three blackhole final smashes. >.>