Yep, either that or a trailer that directly continues the ending of SMRPG is ideal.
I have my own trailer idea, though:
This'll be playing. I'd imagine it to be a Galaxy 3 fake-out. Mario running through the streets of Toad Town, meeting Peach at her castle. Bowser's there, too. A good nod to Odyssey's ending. On the balcony, Mario and Peach make wishes as they watch shooting stars fall, but Mario isn't sure about his wish. It's revealed that in his pocket he has an old picture of the SMRPG group -Mario, Peach and Bowser, but Mallow and Geno too.
They wonder where they are now. Mario sighs, thinking they'll never return.
At this point they spot a star, brighter than the others. It crashes down, making the kingdom fall silent. They look at Bowser, thinking it was his doing. He shrugs.
They go to investigate. Boom, Geno.
<Gameplay trailer>
At the end of the trailer, this song plays:
The group gathers around Geno, a direct reference to the end of SMRPG. Geno walks towards Mario, stopping just a few feet away. He holds out his hand, signifying Mario to shake it.
Instead, Mario pulls him closer, giving him a warm hug. He's happy to see his old friend back.
A "Welcome Home!" banner can be seen in the background.
(I just want my Mario/Geno hug scene, okay!?)