What people really want... is the Mario 64 series... with co-op. (Mario 64, Galaxy, etc.) It's the best Mario series--other than maybe Super Mario World to some--because it has the best level designs and stories.
They tried to half-ass multiplayer with that "every level is isometric and square" one, Super Mario 3D World. But it... it didn't have the charm and fun of the level design. Forcing everyone onto one screen forced the perspective into a less personal one, and much less ambitious and interesting levels.
They added multiplayer to the New Super Mario Bros Wii. Which was good for the multiplayer. But again, the level design felt... boring. I can count the number of "notable" levels on one hand. We felt ourselves playing more levels... just to get them over with, more than enjoying discovering new content.
That's the key thing about 3-D Mario. Exploration. Finding little cute people who tell you stories, and racing penguins down a hill. Fighting a boss that's bullying the town. And the isometric 3-D Mario didn't really have that.
Mario 2-D, I guess, is more about completing a short map, than revisiting it over and over in the 3-D ones. (Except with new power ups to unlock secret areas.) But I'm in serious danger of getting off track here, so I'll stop now.
3-D Mario is still missing real co-op. Not "helper co-op" where the second player is a substantially less useful character (ala Tails in Sonic 3), and usually does not have their own screen.
People want to play games together, and people would love exploring a universe as rich as Mario, together.
You're welcome to disagree. But that's how I feel, and many people similar to me.