I'm almost wondering if DK would've stayed a Switch game releasing around mid 2023 to early 2024 and we would've likely had 3D Mario by now if they had entirely different teams.
In a non-video game example, Walt Disney Animation Studios split up the animation team quite a bit to work on multiple films. Bambi took about 7 years to make because Walt Disney had it made by a smaller team of artists who needed extra training on how to animate animals.

Eh, we didn't get a game related to the first Mario movie either, so why bother making one for the Mario Galaxy movie?

Besides, I'm happy with DK Bananza instead. It brought DK back into the spotlight, and is an overall amazing game.

It was only fair for DK to get another game after Tropical Freeze came out way back in 2014 and the only other 3D DK game came out in 1999. Mario Kart also got its first main game in 11 years.

As much as I would've loved 3D Mario right away, we can wait. I hope Switch 2 repeats Wii's cycle of a 3D Mario, 2D Mario, and another 3D Mario. 

I'm not sure what Nintendo is doing. Nor do I have any guesses toward it of either the gut-feeling or educated sort.

But what I will say is that I'd love to see a Super Mario Galaxy 3, that would be an automatic purchase for me.

I don’t think an outright Galaxy 3 makes sense for them at this point, but just like my op says, I do think it’s gonna be a spiritual sequel to those. Just much more ambitious in scope. Kind of like what Odyssey is to 64.

I wouldn't say it has to have anything to do with Galaxy. Just a Mario game coming out around the same time as the movie is enough of a tie-in. Just cuz the movie is Galaxy doesn't mean the game is gonna be Galaxy 3 or Galaxy-like. I would definitely expect the next 3D Mario game to come out somewhere around the movie release. I mean it was already a HUGE mistake for them to not have the Mario game ready for the Switch 2 launch, so one would think the only reason they held off is to launch it along with the movie hype. The theme of the game though could be anything.

The Mario game to launch near the movie is already announced. Launching Spring 2026. The movie launches April.

I bought and played that on my Switch 2, months ago and had zero issues or complaints. It's a 2D Mario. What in the world could they do to make double-dipping  on that game worthwhile?