With the rise and popularity of fully CGI animated films it's not completely outwith the realm of possibility for a movie done in that style, though not before the live action films have run their course. I don't think we'll see another cel animated movie though, alas. That style of animation has largely fallen out of favour in the US.
I wouldn't be surprised to see something at some point. Marvel does a fair number of straight-to-DVD films; Barbie and Bionicle are just two toy ranges supported by straight-to-DVD films. I think it's less a case of "if" as "when"; the bigger question is what would be the subject of the animation? As the brand is now refreshed almost every year, I would have thought it most likely that a new animated film would be used to launch a new range.
I'd agree with Nigel, that it is likely that an OAV would be used to hype a new range, most probably when the live-action film franchise reaches its natural end a CG animated thing would be a logical next step. (And it will be CG, if for no other reason that by then pretty much all of the cell animation talent will either be gone or locked up in the big studios well out of the price range of the sort of low budget you would be looking at for this sort of thing).