Nintendo is getting close to running out of games they can easily remake. They can remake XY, Sun and Moon, and ALBW for the Switch 2 and can probably remake DKC1-3, but most of their games left that they can remake are going to be really expensive to do a remake of.
The 3DS and DS don't have lineups that lend to easy remakes and they've already aggressively started to mine the GameCube and Wii libraries.
Nah, there are still a lot of potential out there. None of the 3D Mario games have been properly remade in HD form. The 3D All-Stars collection was just remasters. Super Mario Galaxy 2 and New Super Mario Bros Wii are still locked on that system. Sticking quickly with Wii platformers, where is Donkey Kong Country Returns and Wario Land: Shake It?
With Zelda, you still have the Oracle games and Minish Cap. The DS games would probably be a lot harder, but if Nintendo wants to get Phantom Houglass out there, they certainly could. Then you have NES games like Zelda 2 if Nintendo wanted to give that a reimagining. At some point, Nintendo is going to give at least Ocarina of Time the full HD remake treatment. Majora's Mask would probably get the same eventually.
Metroid is dying for HD remakes of 1-4. Nintendo could start chipping away at those slowly.
If F-Zero were to get a remake, it would be GX.
After TTYD, there are still three console Paper Mario games that could get put on a future HD system. Sticker Star can stay on the 3DS.
Luigi's Mansion 1 is an inevitability.
Then you have older GB games like the Super Mario Land, Wario Land, and Kirby Dream Land series. Obviously Kirby 1 won't be enough to justify a release, but Nintendo could do a Dream Land 1 + 2 collection with new graphics and sell that as a bundle. Speaking of Kirby, there is still Epic Yarn on the Wii.
Pokémon remakes are set for the longest time as they can do both the Let's Go Series and the traditional remakes. People forget that Game Freak only averages a traditional remake once every five years.
DS/3DS titles are definitely harder to port given the nature of the original hardware, but there is still potential with the first party lineup. Lastly, I doubt that they will ever touch most of the SNES lineup since that seems to be perfect for all time, but there are plenty of titles to be picked from.