I would work, on a PSX level, and used pre-rendered objects, as with the Resident Evil ( filame thrower, and fire ) and the Paper, or globe backdrops ). Only use 3d if needed. There is too many examples.
About 2.5d. Their is no such thing. 2.5, just means using 3d, and giving it the 2d look. Like strider 2, Arcade. It is a type of look, to mix characters. If so then pre-rendered joy, like on the Saturn.
Finally you mention full 3d. Full 3d is a joke now to everybody and anybody. Their are tons of free 3d programs, with interchangable,
parts. Considering how easy it is for people to get expensive 3d programs, this should not be a problem. Especially in todays world.
I like many other people have 3d projects, we want to work on. I even had an idea for a MMORPG,
Right now their is no limit in videogames, today. You just need to make something and get noticed. Look at crappy angry birds, they were out in the early iphone years, and now every moron probably paid for it. If fricken angry birds can fly then why not a flow-blown MMORPG, or any other kind of game, in any other kind of dimension.
X-box ( yes the original X-box ) is looking real good, with tons of tutorials, and unfinished, projects waiting to get published. So I say follow your heart, on whatever you want to make.