Gonna be interesting to see how Nintendo approaches this; will they kinda dip their feet into the pool or try to go straight for the jugular and try to make a big-money game app that generates big dollars right away?

If it's a big money app, I would say it would probably involve Mario or Mario Kart. We know Pokemon is already coming.

It's probably going to be something I will never play!  

Pokemon Go

"Mario Maker Editor" would be a good move. Prepare some Mario levels on your smartphone / phablet / tablet when you are out and test them on the Wii U when you get home.

It will probably be some type of mario mini games, like there was in super mario 64 ds(if im not mistaken).Though dr mario(a simplified version of it) would make sense.

Metroid: Dread

I'm thinking no matter what style of game it's gonna have to feature their mascot character/s....so I think it would be safe to insert the word ' mario' before it

Metroid U vs Zelda U

They said they want to use mobile to rope people into their console/handheld experience. I'm interested how they want to go about that.

The obvious choice for a first offering would be some sort of Mario game, since they already have their other big hitter, pokemon announced. But that tweet said that we would be surprised....
I like the Mario Maker level editor idea but that doesn't make sense because it wouldn't expand their audience at all.

I also don't think it will be a big rpg (aka paper mario), at least not gameplaywise, this has to be something that can be played in short bursts at a time and is accessible to the mobile crowd. I feel like something like Puzzles and dragons Super Mario Bros edition would have been a good fit, but they are not just going to port that.
Sooo...uh..Wario Ware?

I just hope it's not Pikmin 4......