I would say 2.5D, but seeing how badly the Turtles in Time remake turned out I'd have to say Classic 2D. Streets of Rage needs to be a beat 'em up first and foremost. If that turns out well, maybe consider spinning it off into another genre.
"Now, a fun game should always be easy to understand - you should be able to take one look at it and know what you have to do straight away. It should be so well constructed that you can tell at a glance what your goal is and, even if you don’t succeed, you’ll blame yourself rather than the game. Moreover, the people standing around watching the game have also got to be able to enjoy it." - Shiggy