I prefer 3D games, myself. Though I'm sure they'll take advantage of 2D to make the game exactly like Sonic 3 to make the fans happy.
I actually am looking forward for more 3D Sonic, I got indeed, very happy with Generations Modern, and when I saw today Sonic Dash with the Spindash, I feel as if their getting closer and closer to a style of gameplay everyone can relate.
That said, my personal preference shall always be 2D, that's how I got to know the blue blur and even today I feel as if that's the game I want to see happen the most, it has a whole lot of potential and there's quite a fanbase waiting for it. I believe giving 2D Sonic a chance to shine will provide to be very awarding for Sega as would for fans all around the globe - I truly, truly hope for Sega to achieve such a game.
Personally, I think it would be cool if instead of making Episode III, Sega should start fresh and make Sonic 5. Sonic 5 should have Taxman (from Sonic CD 2011) design the graphics (and physics) in smooth, high-definition 2D instead of S4's "2.5D". It would kind of look like that Sonic 2 HD fan project. Sega should also take take the liberty of making 16-bit sounding music. The classic character designs would also return.
You should be able to pick between Sonic, Tails and Knuckles (who would be in this game) to play as. Sonic 5 could bring back the elemental shields, and even introduce some new elemental shields to make the game feel more original. I would also STRONGLY suggest they get rid of Sonic's homing attack, and replace it with the Super Peel-Out. This game should NOT be episodic, but one big game, with an act amount that outdoes S1, S2, and S3&K combined. Today's technology should be capable of this. The game should also be available as a physical disc that you get at a store (like the "good old days") instead of just being available for download. Online multiplayer would also be nice, if it is done right.
I think that Sonic 5 should continue the cliff-hung story of Sonic 4: Ep. 2 and possibly even lead into Sonic Adventure 1 to finally bridge the story gap between S3&K and SA1. Knuckles Chaotix could also be re-released and have it's events act as a prequel to this game.
I know I'd be getting my hopes up too high, but I HIGHLY recommend Sega to make a new 2D game like this. Now THAT would be "Sonic as we truly imagined it"!
SEGA has far more 2D titles than 3D titles, and most of the 3D games (and even the 3D stages of Sonic Generations) have at least 50% 2D sections.
As much as I appreciate Seeing 3D Sonic games going 2D every now and again, It doesn't really feel as if we were playing a Classic genesis game simply because of how much Modern Sonic's abilities differ and how the levels are built around him. I like it that it's there, but it just won't be able to provide a classic experience altogether.
Giving 2D Sonic a chance will be beneficial no doubt, as Modern Sonic keeps evolving into an even greater game with its own identity, so too can 2D Classic games make a comeback.