In a pipedream..
I can imagine a faux open world Mario game that progresses like a traditional Mario game, but has stages like 64 or Sunshine. So to start, imagine 3D World controls/mechanics with all of Mario's parkour ish moves from the early 3D titles.
Alright so the general idea: Mario has to defeat Bowser at his big castle whatever thing way at the other end of a big map. There are multiple worlds in between you, but the amount depends on your playthrough and what exactly you do in each world.
Let's say you go into stages roughly the size of Mario 64 levels and do many missions for power stars, all whilst interacting with NPCs. In certain specific missions, you hit story beats that give you the option to take a path to the next world (which'd be naturally / seamlessly connected). But the thing is, because it'd be treated more like a geographical location than a regular Mario game would, you might have more than one ending you can get for a single stage, with that depending on who you talked to or what missions you did.
For example, the grassland / Mushroom Kingdom level at the beginning of the game may have three different paths that may lead either into the jungle, further into the Toad villages, or lead you to the desert. From there, each of those paths may have up to two or three places they can lead to as well (possibly even a shared area or two in the middle of them), and that would continue for a several more worlds until it converges at the end at the big bad Bowser location.
This'd make for a Star Fox esque 'carve your own route' kind of gameplay where there are numerous different paths that can come out of your playthrough depending on how you approach the game. Also since you'd want to collect all the power stars, you'd want to explore and do as much as possible in the game to get them all, backtracking and covering all the land in the process.
Also fast travel = warp pipes. ¯\_(ツ
_/¯
But yeah that's one idea on how taking the Mario series forward could work. Not saying it's perfect, but it's a concept, and I wouldn't mind seeing Nintendo pull something similar.