This October it will be 6 years since Mario Odyssey, ordinarily we would already have gotten the next 3D Mario, which makes me strongly suspect they're holding onto it as a first year game for the successor.

Mario Kart 9 should also be very far along by now if Nintendo have any common sense.

I am skeptical if Nintendo will want to entirely let go of their 120 million plus Switch userbase right away though, specially considering the 3DS saw support for a 2 years after the release of the Switch.

They are either sitting on a new 3D Mario for this fall because of the movie or holding it for the launch of Switch 2.

Mario Kart I think is the most important Nintendo IP, even eclipsing the Mario platformers at this point, MK9 (or 10 if you count Tour as 9) has to be deep into development right now unless Nintendo has lost their marbles. I would want that as early in the Switch 2's product cycle, even day 1 if possible (since you're not going to have a new Zelda). 

Even Animal Crossing and Smash Next, like I would want those to be in actual development by now, not just sitting around a table and discussing ideas. That should have already been what those teams were doing over the COVID pause.

As an avid 3DS gamer I'd say the transition to the Switch was formidable in any case cuz the reality of it is that the WiiU was basically dead by the end of 2015. So the transition to that took them more than a year, even then they had to use part of their previous library on the WiiU to make up for the holes it could've had in it's first to second year schedule.

The GC to Wii had actually quite the smooth transition too going back from memories because they actually decided to make the Wii into the GC + basically. To me in the beginning, these two consoles were almost indistinguishable.

On the handheld side, the whole era of GB to GBA and finally DS was almost flawless

Yeah for the most part their handheld transitions have been more seamless in that the old system isn't killed off prematurely and even lives on for a while beside the new one, yet without depriving the replacement of support and system sellers.

Even 3DS continued to get games well into 2019.

Actually, even during 2017-2018, my time spent on games was still mostly done on 3DS. It was truly once the support had dried and that games on my Switch began to pile up where I truly fully transitioned.

I know this prolly won't replicate itself with the Switch successor because I'll get it on launch most likely but they certainly would hard pressed not to find a way to make the transition from Switch to Super Switch just as smooth as those with cross-gen titles obviously.

The next Mario Kart is 100% a game that needs to happen in year 1 of the successor, it's the biggest no brainer, and they've had enough time to make it happen.

If the successor launches next year, it should also be viable to get the next Animal Crossing out in the first year. 

Throw in 3D Mario and you have a first year to rival Switch's 2017.

It depends on the criteria for it. If you mean based on a rise of gen over gen sales, Wii U to Switch. If you mean the smoothest transition, probably GB to GBC, or GBC to GBA, GBA to DS.

If we’re going for personal feelings, my favourite was Gamecube to Wii, as it went from one of Nintendo’s lowest points to one of their highest in a very short time. How it personally impacted me was it brought me from a time when I wasn’t really much into gaming anymore (at least, new games), to being back like Casino Royale.

Bowsette is another way of looking at successful transitions by Nintendo.