When is Mario Maker supposed to come out anyways? And on top of that, why Lucina and Mr. Game & Watch if their involvement in Smash was merely coincidental to the segment itself? Why were the only bubbles of characters that, at the very least, appeared or will appear in some capacity in Sm4sh? And on top of that, why is 8-Bit Mario currently the only non-playable character in Smash 4 who hasn't been disconfirmed as of yet? And it's not just Mario in his 8-Bit form, either; Mario in his 3D model is there as well!
I may be grasping straws, but I am not about to disregard this, especially since, again, you put it in the same context as ROB and Mr. Game & Watch, it makes a ton of sense that he'd be playable. Sakurai is rather coy in his hinting of characters; so far, Shulk, Mewtwo, Jigglypuff, ROB, Falco and Wolf are the only characters so far not to have been hinted at all. And guess what? The Pokémon trailer showed Mewtwo and Jigglypuff completely absent; Monolith Soft practically gave Shulk away the weekend before the Fire Emblem trailer took everyone by surprise; and if you want to count the Arwings in last night's potd, it can arguably be regarded that as a coy hint towards Falco's return. If you wish to apply all of these little details, it pretty much means that ROB and Wolf are the only characters not to have any hints pointing in either direction.
You can call it straw-grasping; but it shouldn't be so easily dismissed because it's hard to imagine his inclusion.