I'm not buying a new switch model this year. The 'pro' has been coming for 3 years now.

Odyssey 2 would likely have been a Switch game so a new 3D Mario on Switch 2 will be a new title.

The 4K remasters are a pipe-dream as Switch 2 is simply not going to focus on that level of graphical fidelity. 2k may well be a stretch.

Pokemon will likely come in 2025 as that tracks with the consistent schedule of new mainline release every 3 years which they've followed since 2010.

Ever since the Switch's release year people have been saying they used up all their big guns and they'll have nothing left to release, now we're just moving this conversation to their next console?

Sales have declined, yes, they are still very, very healthy though as they were insanely high. They just released a sequel to a launch title and, on top of the console being the highest selling for the month, it became the fastest selling exclusive of all time. (for comparison God of War and Spider-Man sold 3.1 and 3.3milion first week, while TotK sold 10milion) There is no predicting what games they might have in store that can boost sales. There's also no saying that sales will drop at the same rate next year, maybe they manage to stay flat.

We still have a Peach game, Luigi's Mansion: Dark Moon HD, Metroid Prime 4, Professor Layton and Fantasy Life as confirmed for 2024 or beyond. They can add a new Kid Icarus Game, F-Zero, New IP, Donkey Kong Original Trilogy Remake, Ring Fit Adventure 2, Nintendo Land 2, Arms 2, New Warriors spin-off, TotK DLC, Pikmin 4 DLC, Mario Wonder DLC, Astral Chain 2, Pokémon Legends:, 2D Zelda (Remake or New), some 3rd Party exclusives, Mainline Pokémon, etc.
The point being they have plenty of options to even fill 2025 with decent games without jeapordising the launch of a new console.
Introduce a new console during holiday 2025, combined with a massive price cut for the Switch and the release of the Nintendo Selects line. Release a new console in march 2026 with full backwards compatibility and improved performance patches for switch games.

They can launch their new console with:

3D Donkey Kong action adventure
new gimmick game to highlight the console/controller capabilities
Mario Kart 9 (or 10/11 if they count Tour and Live)
---------rest of the launch year options---------
3D Mario
Smash Bros
new IP (Monolith)
Paper Mario
Zelda Spin off (Zelda main character, OOT/TP/WW/SS style)
Luigi's Mansion: Ghost Town
Super Mario Maker 3
Fire Emblem
2D Metroid
Pokémon Remaster
Star Fox
Kirby
Yoshi

Now, I'm not saying this is definitely going to happen like this, just that there are other options besides ¨Console sales show decline -> release new console asap¨.

Game Boy had home consoles alongside it and an explosive new IP in the form of Pokemon that launched in Japan in 1996 and the rest of the world a few years later. Switch has neither, which means riding Switch out for another 3 or more years before replacement is very risky. 

And if the dev kit rumors are true, that all but guarantees Switch 2 is coming out 2024 or at least 2025. 

Nintendo said they would struggle to sell over 10 million units this fiscal year, likely because they are planning to announce the console successor this fiscal year. Especially considering the unit sales are as high as they are still. Nintendo realize an announcement of Switch successor will crater Switch sales.

This is what makes the speculation different this time... Nintendo's confirmed comments allude to new release. Another example is Ubisoft saying Nintendo told them to hold the Rayman sequel to the next console. Does anybody thing Nintendo would ask Ubisoft to sit in a game for over 2 years?

1) It was 15 million units. And likely now that we see how big the hardware sales have been this quarter with the release of Zelda and the last Direct announcing Mario bangers for this Holiday, I think the discussion of an announcement for a successor is void.

2) Not Rayman but Mario + Rabbids Sparks of Hope. 

Have we already seen in the past a big console release in a summer?

No they projected 15 million but said that might be a stretch. Biiiiig difference between 10 and 15 million. 10 million means successor is imminent, 15 million means system is still welling well but is getting late in its life. Plus they just sold 1.5 million in May, so sales are picking up from when they said 15 million would be hard to hit, and at least according to Japan numbers sales are still going great in June. Sales will of course slow back down, but considering current sales, plus a holiday season, plus Mario Wonder, 15 million seems very likely.

Anyway, as to the question of the thread, next gen first year titles (whether that is 2024 or 2025):

The two obvious ones are 3D Mario and MK9. Those have been the obvious launch games for next gen for a few years now. Prime 4 should be a launch period title if it turns out Nintendo moved it from being a Switch title to a next gen title so that is a pure maybe. I expect alongside 3D Mario on day 1 for them to do another little party game since Nintendo likes to do that though hopefully that quality of it will be more in line with Wii Sports rather than 1-2 Switch. That would do for a late year launch, obviously more games if its an early year launch. If the launch is 2025 then next mainline Pokemon game would be in the first year lineup since the Pokemon schedule has that coming holiday 2025.

Basically:

Most likely launches -

2024 (Fall launch): 3D Mario, little party game, MK9, Prime 4 if not already released

2025 (Spring launch): 3D Mario, little party game, MK9, Prime 4 if not already released, Pokemon mainline, one or more likely two (probably AA) titles that would fill in the space between the big launch period titles and the holiday Pokemon game. So this schedule could be something like 3D Mario and party game day 1 in March, MK9 in April, Prime 4 (if it didn't come out on Switch) maybe in May, AA game in July, AA game in October, Pokemon mainline in November.

Potential other/AA games in the 2025 schedule I could see being any of these: Yoshi, Kirby, Luigi's Mansion, a new IP, something that didn't come out on Switch like Star Fox or Kid Icarus, Paper Mario, maybe a new 2D Zelda or just Z:WW and/or Z:TP port.

I'm really hoping for a 2d Metroid and more Splatoon game in addition to the guaranteed franchises that appear every generation.

I expect the Switch 2 to be out near Christmas of 2024

We have, whether in NA or other regions. I think it's been a while, though. 

N64 launched in June 1996 in Japan. Game Boy Advance (a portable) launched in June 2001 in most major regions. SNES was an August 1991 release in NA, while Famicom was a July 1983 release in Japan. Game Boy launched July 1989 in NA. 

PS1 launched in NA on September 9, 1995. That's still summer based on science, but many countries consider summer over on September 1. 

But it seems Q4 or Q1 releases are best for hardware, whether it's a home console or handheld.