You guys seem to be working along the lines Sega went with for Sonic Generations, and I like it.
I'd like 3D Mario games with different themes. Sunshine has the tropical vacation thing going on, Galaxy is set in space, 3D Land takes after the early 2D games. I'd like:
1. An underwater-based Mario game - Swimming's usually problematic in these games and this would need to be addressed. Mind, it need not take place solely underwater, but always with a water theme.
2. An urban city-based Mario game - Think Grand Theft Auto. Come to think of it, I can't recall an urban city ever existing in Mario's universe, and if one were created it'd have to be uniquely Mario.
3. A Nintendo theme park-based Mario game - This would be like the perfect love letter to all of Nintendo's history. Until that day when Nintendo actually creates a theme park, that is. This idea somewhat inspired by Sonic Colors.
4. An afterlife-based Mario game - HaHA! Mario's dead. In his absence, Bowser's taking control of the Mushroom Kingdom, and Mario must set out to collect the 120 stars in Nintendo heaven to get back to the land of the living and dunk that sucker in lava!
Also, I certainly wouldn't mind:
5. A Metroidvania Mario - You start off as Mario and set out to rescue the rest of the playable characters: Luigi, Wario, Waluigi (essential character, in my opinion), Peach, Toad, a female plumber, and others, each having different control schemes and abilities and allowing the player to unlock more of the 2D open world.
6. A loveletter to the series's history Mario game - Since you guys brought it up, this idea sounds great.
EDIT: Sorry - after posting, I picked up on the "2D" part of this thread. Well, at least I gave a couple ideas at the end there.