Rosalina supporter must be having one hell of a party right about now. I'm just going to say this, right here, right now, to every Rosalina supporter:
I was wrong to consider Rosalina a non-contender.
Even though I did thoroughly enjoy the character, I never expected her to compete with heavy-hitters in Toad or Bowser Jr. I'm glad to say I was wrong.
If I were to venture into guessing a Final Smash, I would suggest a hailing comet storm within a designated area (perhaps within a trajectory range of about a 45' and/or 135' degree angle), hailing down meteorites of Lumas and bringing awe-inspiring destruction in their wake. It's only my speculative take on it, so I'd use it with a grain of salt.
Otherwise, I'm genuinely ecstatic at the confirmation of Rosalina.