While I'm here I might as well raise a point, as to why I think he might be playable,but I'll also do a little counter-argument against him being playable.
Going by appearances in the games Other M (Which his model will most likely be based on, that or His concept art for that game) Was on of him Smallest appearances, and yes I realized the size argument shouldn't be a thing, but some people still think it is. One of his Biggest appearances was in Brawl, he was huge in brawl. As to bring up another thing, Bowser isn't that much taller than Mario in smash, in the Mario games however, 2-3 Mario's =One Bowser. In Samus's Games depending on the game, Ridley is only 1 Samus and a Half tall, sometimes 2, but never more, except Brawl, where is was indeed around 3, maybe even 4, but as I said biggest appearance. Also it worth mentioning they could also pull at the fact that Ridley has been altered and changed so much in the series that Ridley could be playable, and Meta Ridley Could be the hazard. That wouldn't be an incredibly farfetch'd conclusion, though it is unlikely, it's plausible. Hell if they needed to, which they don't they could pull a Alfonzo. When Ridley joins the battle: *Insert other boss here takes his place*
2 more points.
Ridley didn't look like a Boss in the clip of him we saw, he wasn't attacking, he moved doing (Judging by shadows is hard but) what looked like a Jump or two, and by the clip, is seemed like he has(if playable) a float ability similar to Peach's. If he were a Boss he movement would also be a lot less Jagged, and Rough. I mean look at Yellow devil very smooth in how every animation is executed. while with Ridley he seemed a lot less smooth, unlike that of a boss.
Also I could be wrong with this but In Metroid: Other M. Was that the same Ridley, as, he Had an Adolescent Form, meaning he had been a child, but in previous games he was fully grown. Which according to the timeline shouldn't be, as Other M is second to last on the Timeline followed by Fusion which had an Adult Ridley once again. So couldn't it be so that This "New Ridley" was the Boss, and the Old Ridley is playable? *Note I haven't Finished Metroid Other M, I have beat Ridley at Pyroshere though.*
That my two cents... I'm not sure If I'm repeating whats already been said as I haven't read every single post in this thread, but thats all I had to say.