http://www.dualshockers.com/2013/11/03/open-world-super-mario-possible-if-fans-ask-for-it-3d-world-wont-have-dlc-gamepad-used-extensively/

I don’t know if we can consider Super Mario 64 as an open world game, but I think Super Mario Sunshine is more consistent with that definition because of the city and entire worlds to explore. Our main standpoint with Super Mario is to ensure that players always know where to go.

It’s for this reason that we have given great importance on the placement and visibility of the flags, particularly in Super Mario 3D World . Again, as creators, we want a large number of people to be able to play and enjoy the game, so it’s sort of our duty to ensure that the objectives are clear to everyone. However, if a very large number of players expressed the desire for a more open Mario, we would take that request very seriously.

I say they should go for it, a more open-world like Super Mario game. EAD Tokyo has done 4 linear styled games already, I think they should try out something new. We've never seen them tackle an open-world Mario game, and I'd love to explore the lands of Mushroom Kingdom. You heard Koizumi-san, let's express our desire to have a more open Mario game (those who want it). 

That would be pretty awesome, but I want a Super Mario 3D Galaxy for Wii U next. That type of game should be for Nintendo's next console

If they are gonna make another mario sunshinesk thing, then no... I did not like that game very much if at all... The only part that was awesome were those stages with the crazy platforming... But everything else was dumb imo... If its more like Super Mario 64... then go for it... Loved that game

Well, I thought the world had already been asking for a 'Super Mario 128' ever since 64 came out and that GameCube tech-demo...

:-/

head to Miiverse to ask for it!

I think linear mario games are lazy, so its good they take fans into consideration. Maybe an open world mario galaxy type game shoould be next

Well... I would be interested how we gamers can express our desire for an open Mario game, because we tried for the last 3 years and it seems like it didn't work at all xd

According to this interview, that could be the case.

"Koizumi: Well, this is my personal opinion but I think that with all the experience accumulated by the teams of Mario or Zelda, it would be difficult to fully leave a key episode of the series to external studios. However, as we have found ways to collaborate on projects with other companies, it would probably be possible to do the same thing with Mario finding a truly talented and motivated team.

On EAD Tokyo moving on to a new project

Koizumi: I can not tell if it's a Mario game, but I can confirm that it works well on a new project."

Haven't fans been asking for it since the Wii U came out...

when I think Open world mario for some reason i imagine Mafia III with Don Luigi vs don Mario.