I'll admit I'd love to play another 64/Sunshine-style open world Mario game, but in 3D Land's case, it's like what Galaxy SHOULD have been.
A Super Mario 3D Land 2 could potentially use the same engine and general workings behind the powerups, but nix the timer and build more nonlinear levels like 64 and Sunshine with Power Star missions. The problem is deciding how to distribute powerups around an open world without having them respawn indefinitely and run out of time, because the player would for some reason expect to be able to go back and get it if they lost it even though that's not how it works in the 2D games, you know... Leaving ? blocks littered around everywhere with powerups functioning exactly as in 3D Land might be worth an experiment, so long as they were never required to get a Star.