A crossover in a crossover?! Imagine if they shared Labyrinthia as a stage!
I know it's obvious but I'd love to hear a Smash version of Professor Layton's Theme. That would be SO GOOD. Get an electric guitar for that groovy bit in the middle, can't really describe it in text but I'm sure you'll figure out what bit I mean!
Would be so hype in a reveal trailer too. Nobody could be mad with heavy rock version of that playing in the background.
I also really, reeeeaaaallly want the Theme of the Last Time Travel in Smash - probably my favourite composition in the series. I was kind of gutted with the Persona music in Smash (though Persona 5 in particular has a lot of bangers and I know they can't add them all!) so I'm not getting my hopes up, but if we live in a timeline where Layton gets into Smash this would make it the definitive best timeline.
For his costumes, I wonder what route they'll take - will they go for inspiration from Layton characters, or will they base it on the main colour of each game (red for Pandora's Box, green for Lost Future and so on) like they did for Persona? I know Persona had a couple of references to other characters but you know.
That wedding one would be a great bit of fan service of notched-ear Pichu proportions, I'd love that. What are the chances of Inazuma 11 Layton?!