I say Metroid Prime 4, though I think a Zelda Remake is coming sooner than we think.
Reasons? Complete speculation.
Tears of the Kingdom was essentially done in 2022. It got another year in the oven just because they could. Had COVID not happened, it likely would have been finished in 2021. Usually while they are finishing one Zelda game, usually another is already in the pre-production stage.
The last time we got a Zelda remake was in 2019. 5 years is a long time for a remake to cook.
Now it's possible it could be a surprise game for the Switch this year, something like a Link to the Past redone like Link's Awakening. But I have my doubts about Grezzo doing it, because they have been busy with other projects over the past few years (nothing big though).
I find it more likely that what began as a somewhat typical 2D remake beginning in 2020 evolved into something much bigger.
Around that time, the rumors of a Pro console were running rampant. As we know, that never happened, instead 5 years later we are hearing of the successor to the Switch.
If there was a remake early in development, around the time a decision was made to scrap the Pro model console and focus on a successor. Likely that game would move to the successor console as well.
It would make sense to have a Zelda game near the launch of a new console. I doubt Nintendo ignores the fact that both Twilight Princess and Breath of the Wild helped usher an environment for 2 of their highest selling consoles.
At the same time it's unlikely to be a launch game, but more likely a year 1 title.
So I figured the main culprit would be Metroid Prime 4.
It's never had the type of major selling status as other series Nintendo has, but it is in a somewhat unique position. Almost like Twilight Princess, it's a game the hardcore have begged for over the years. It's been in development for a long time, and a good part of the development was for the Switch. But positioning it as a cross-gen launch title (sadly, I wish it was just for the next console but I have my doubts) is probably too tempting to ignore. It could perhaps ignite the sales for the title and spawn excitement for the new console while simultaneously not disappointing people who were waiting to play it on the original Switch.
More than likely, the game was delayed to increase the graphical quality of the version on Switch 2 and/or take advantage of some new quirks to the controller/console.
I think a 3D Mario and Mario Kart 9 have been in the oven for so long they are more than ready, but Nintendo has learned it needs more than Mario to succeed.
MonolithSoft's next game I think is much farther off, probably 2027.
Though I think Next Level could have something soon, it would be really nice to have a new Punch Out game.
And Donkey Kong, well who knows at this point. It's been over a decade since he had a new legit game.
I think 3rd party's have been more than ready for the next console, and Capcom probably has something special for year 1.
All speculation, but hey. It's why I made the thread.