Yeah, having Rosalina and Geno in the same trailer would be really great but I don't believe that this is going to happen.
What I would love:
Peach is with Kirby, they try to do something about Mario, Mega Man, Dedede and Luigi who are in their trophy form after getting defeated in trailers. Bowser attacks them with his army. But a light is seen by everyone in the sky while we're at night. Suddenly, there's a big flash and when he's gone, Mario, Mega Man, Dedede and Luigi are back to normal. They're like "wait, where are we ?". Bowser doesn't care and attack them but a HUGE projectile knocks him back. Peach and the others don't understand, they hear a sound in their back and then... Geno comes out of the obscurity and he's serious business ! Then, we see some gameplay.
Bowser is shocked. Geno charges a huge attack, Bowser and his army run away (Bowser knows about Geno Whirl). Then, Luigi is confused, Dedede and Kirby celebrate, Mega Man don't do anything while Peach and Mario look happy. The trailer ends with Geno, finally smiling and watching the camera like "I'm back".