2D all the way. The fighting game is the most direct and bloody test of skill between two players. It should be about their skill with the characters, use of block and special moves. Two enter, one leaves.
The worst thing to happen to fighting games however wasn't 3D. I don't like the extra dimension but it doesn't ruin the game...just needlessly complicates it, adding another axis into the control scheme. The worst thing to happen was the "Super Hyper Mega Moves". Street Fighter IV was so horrid because of this one feature, with no way to disable it. As you get hit, block and attack, a super meter charges. When full it allows the use of a ridiculous ability that obliterates the opponent.
That defeats the whole beauty of the fighting game. It is no longer a game of skill, a battle of reflexes and reading your opponent. Further, a losing player's super bar charges very quickly and almost always grants a use of this power that brings the better, superior, dominant player from full health to near death. This is cheap, cheating, dirty, horrid and stupid. I hate it with a passion and wish these super moves would be removed entirely, or at least disable-able.
Super Street Fighter II Turbo was the ultimate fighting game and to this day I cannot name another better than it. A large roster of well loved fighters, each of whom had useful special abilities and unique twists (dhalsim range/teleport, sagat high/lo fireballs, chun li/vega jump off walls, zangief strength, balrog dash, etc). They were all reasonably easy to learn but difficult to master, the perfect level of challenge (WTF is up with 20+ special moves per character now?). In the hands of a good player, any character could be deadly. Also, 4 hit dragon punch ;-)
If Capcom want to make the best fighting game of the next generation, all they need to do is remaster and re-release the above.