Makes no sense. Numbers don't back it up. Since the restructering Nintendo has become more efficient when it comes to 3D Mario games. You don't start this early making games for the next console. That's like saying every developer that has recently released a game will now be focusing on Nintendo's next console. That would be suicide for Nintendo as it would alienate their last fan stronghold. 

Super Mario Sunshine and Super Mario Galaxy were not made by the same people. They didn't start development of Galaxy right after Sunshine. Unless you're suggesting they spent 5 years on making Super Mario Galaxy. 

They did however start working right away on a new Super Mario project after Super Mario 3D World. Game has probably been in development for a year already. 3D Mario games usually take 2-3 years of development time. This game could very well be done already in 2016. The third year of the Wii U life cycle. 

I just can't see them devoting two full scale 3D Mario games to their least selling system, unless it's a 3D World 2 that recycles a lot of assets.

(Which would be a huge disappointment)

IMO an 8th generation evolution of the SM64 world is the definite way to go, although this should be a huge open world, lots to do, with no barriers, paintings can be used for warping to areas of the world (maybe houses can be all over the kingdom with them in and they'd have places to save), but make it so you can walk to those locations if you want to, use Yoshi to travel there maybe, like Epona in Zelda.
SM3DW isn't what most gamers would class as Wii U's big Mario game, this next game shouldn't require you to use maps to get to each new level you want to complete, although the warp paintings or maybe plumbing pipes could be used as shortcuts to get you all over the Mushroom Kingdom quickly if you want to speed up travel around the world.

No reason why you can't have huge platforming sections in the skies, with cloud sections to the world that you can explore.
There could be underground dungeons to explore, other kingdoms, water locations.

Make this a 2016 release, save the next Galaxy game for the next Nintendo console.
I'd love this and I think Wii U needs a game like this!

I think people often forget that the reason why Gamecube and N64 only had one 3D Mario game like this is because they had their big Mario release, I don't think Wii U has had that game yet.

This is what I think as well. 3D Land/World are kind of like their own sub-series. I believe the Wii U has yet to have its real, main-series 3D Mario game. 

Actually, I would say the slow sales would be a motivator to have one more big Mario game, especially something as big as Super Mario Galaxy 3.  Attachrate  is everything with a slow-selling platform and Galaxy 3 would both move consoles and have a very high attachrate.  Plus it would make a lot of noise in the community/industry, which is what Nintendo needs to do themselves since they lack 3rd party support. 

I think Mario should go back to his roots.

No galaxy. No tropical island. Instead, a gigantic hub world set in the mushroom kingdom. The idea is that you can traverse different parts of the Mushroom Kingdom (ones that haven't been explored yet). In many cases, similar to Mario 64.

I'd like them to show us a Mushroom Kingdom we've never seen before! Although, at this point, that might be tricky lol.

I would love to see Galaxy 3 on Wii U, but I think Nintendo is going to be cautious about investing in many more big-budget Wii U titles, bar ones like Zelda, Xenoblade Chronicles X, etc that are already well underway.

I would love to see a Mario 64 sequel, but it's not going to happen...

Super Mario Galaxy only had 60 people working on it. Super Mario Galaxy 2 only 40. Xenoblade Chronicles X will sell far less and they still made it anyway (they knew this even before the Wii U's bad sales). 

Wii U really needs a true, new 3D Mario. 3D World was an offshoot of 3D Land, and it was more a 2D/3D hybrid. Something like Galaxy 3, but with a more open-world design closer to Sunshine and 64. Galaxy 1 and 2 were too linear.