Is it unrealistic to hope for single-player only? That way the levels can feel tighter and better crafted.
I'd love it if they get more creative than the grass-sand-water-rock-lava template they've been using since forever.

Also, good music.

The 3DS one wasn’t poorly times, it was simply not up to par. It was arguably the worst 2D Mario game since Gameboy. It was too repetitive. The Wii U one is actually fairly good, and one of the most underrated in the franchise; if any game suffered from poor timing and a poor platform, it was that one. Also, one major draw to the franchise is the multiplayer, and NSMB U felt more like a single player game because everyone wants to use the Gamepad, which really hurt it. Nintendo’s strength with these sorts of games is the multiplayer.

Having NSMB U ported would be a great idea, but it should be rebranded since the title makes sense and is a little on the toxic side of marketing.

For a new game? Nintendo knows best, but I would love to see some of the design philosophy of Super Paper Mario/Yoshi Switch make it in. The fact that everyone can have their own screen will make multiplayer far stronger than ever before, add in the multi-layered SPM/Yoshi design, and that will potentially be the greatest 2D platformer ever made.

I really loved the first two New Super Mario Bros. games and still do...
I played the other games to completion but was unable to appreciate them as much...
Neither New Super Mario Bros. 2 nor New Super Mario abroad. U felt ambitious enough, and New Super Luigi was really just a chore...
I think different art styles would be kind of cool...
The NSMB games feature different world as the player progresses; perhaps in the next installment, the art style could change from one world to the next...
For example, the first world could look like a traditional Mario game, the second world could look like an impressionist painting, the third like a chalkboard, the fourth a comic book, the fifth a stained glass window, and so on...
If that's too ambitious, then how about just worlds that aren't generic plains, desert, snow, and water worlds???

I think the rumors will end up being wrong like the early Smash rumors that suggested it would be an enhanced port rather than a new game. Btw, I agree with most people on the thread: just give it a new paint. A new, cooler artstyle with super clean character models like Odyssey would give the game a new personality. A more coherent world map and areas would be great as well, like people said before. Add completely new songs on top of that and we're all set.

The level design is still great, so they could just add interesting new power ups and modes.

Change the art style. I would love to see a Mario game look like Rayman or Shantae. Two really odd artsyles, but I think that they could work.

A completely new art-style(2d,in high definition, not pixel art), and a name that do not resemble to any previous Mario.
New elements and power ups, but bring one of each Mario series. Keep multiplayer, 8 this time, bring Mario, Luigi, Wario, waluiggi, toad, peach, Rosalina, even Pauline as playable. and yoshi and birds as mounts.
Add online features like splitscreen races or coop.
The rest, let Nintendo magic begin.

2 years latter, launch super Mario maker 2 with this new art style (and maybe the 2d parts of 3d land/world) and features like buttons to change art-style/physics middle play, vertical stages, splitscreen racing, ghosts of best plays(appearing while you play, or even when you edit. World editor. And continuous support with stages and world's from original game designers.

I hope the nsmbu doesn't change these plans.

Either Super Mario Bros + Maker (which should be both a full new adventure and have everything from Mario Maker Wii U), or 'Super Mario Bros 4/5' (which should be equally inventive as the first games).

The greatest evolution of 2D style Mario game was with Super Mario 3D World (which isn't a real 3D mario), they should go with that

Make it a true 2D game with hand drawn sprites and detailed backgrounds. Make the physics tighter, not like the ground is made of ice like in NSMB. Keep multiplayer.

Super Mario Maker should be a separate mode attached to all future 2d mario games.

Also they should move back to 2d sprites, NSMB has been done to death and makes me sick.