Exactly. When I saw it live I thought something was incredibly weird with his animation. It does indeed look like someone is controlling him. Still that shadow does look too big for playability but it could also confirm one of my worst fears. Special playable boss battles.
What ever the case it does seem there is something more than just hazard boss Ridley.
That's not a terrible idea IMO - you could make whoever has the Gamepad be the boss that controls the stage (item distribution, stage hazard appearances, speed of battle etc) as a 4 v 1 battle, could work for Master Hand/Crazy Hand/Tabuu, but it's less likely for Ridley/Rayquaza/Petey Piranha etc.
Edit:
Zero since Dark Samus's an Assist Trophy (see below).What are the chances that at E3 we get a trailer announcing Ridley, then Dark Samus comes in and blasts him off the stage. "Dark Samus Consumes the Competition"
I'd accept a small delay for the inclusion of Dark Samus