Well, about that:
This was a construct made by Vaanrose using all the stuff we had learned regarding stage placement. Miiverse is incorrect (DLC stage post-release) so that's as blank as the stage on the second row, meaning there are upwards of two missing stages on rows 2 and 8.
One possibility is that there are no further remaining stages at all, and that rows one and eight have five as opposed to six stages, but that would force Delfino onto the second row. The rather perfect stage organization of the above conflicts with that, as the Mario series seem to have stages separated as follows;
2D platformer > 3D platformer > Mario Kart > Misc.
This technically debunks any returning Mario stages at all since it would force jumbling of rows, and makes a Donkey Kong stage the most likely outcome. It's unlikely to be a Party stage because a MP9 track appears on Mario Circuit 8, and it's unlikely to be sports related because Mario Circuit Brawl has a Tennis/Golf sports mix theme on it.
The stage ordering itself is as follows;
64 Series > Melee Series > Brawl Series > Smash 4 series > Retro > Misc > Third Party.
Port Town is unlikely to move up to give room for row 3 because its placement has consistently across all stage select screens been between Metroid stages and Yoshi stages, and FZ2 by series order would need to be with Fire Emblem. So the main thing to get is that either it's a VERY off-the-grid/unexpected Mario Stage or it's a DK stage that's either Kongo Jungle Melee or a new DK stage. The fact Kongo Jungle as a past stage has "64" in it may actually be a bit of a clue.