The only (terrible) bad 3D Sonic game was Sonic 2006, it was horrific and unplayable.
I don't see anything wrong with Sonic Heroes (I thought it had a unique twist on the standard gameplay), Shadow (So what he had a gun? The story was pretty intresting) or Secret Rings (Yes, I know many of you people from ScrewAttack back in 07 thought I hated this game and I did, but I gave it another try and I can say it is respectable in the style of the gameplay).
If they do make a Sonic Adventure 3 (which they won't, because the story arc has passed. But if they did hypothetically speaking...) it should be like Sonic Adventure 1, but better: 3 or more Adventure Fields, up to at least 20 Action Stages altogether, bring back E-102 Gamma and Big the Cat (Gamma had a very intense storyline for a Sonic game back then, involving killing off his brothers to save them from being dismantled and reformed into freakish war machines, he also had a very though provoking ending. Big the cat's gameplay may have been a drastic departure from the standard, but it makes a nice relaxing change and gives you time to take in the scenery, especially on Emerald Coast). Station Square HAS to return with the original or redone music, add the 3D Green Hill Zone from SA2:B, or heck even a whole 3D Sonic 1 (I mean if SEGA really wants to keep Sonic alive, they better show their fans that they mean business, take Nintendo for example: They don't fuck around when it comes to Super Mario games). And it should NOT have the 4Kids voice cast, I know this argument has gone on for years, but come on! Ryan Drummond IS Sonic The Hedgehog. I know they can't bring back the original Sonic Adventure 1 Dr. Eggman voice actor Deem Bristow (whose tragic death cause the whole 4Kids takeover), but seriously listen to Eggman speak in SA2 or any other game after and then hear him ROAR with his amazing voice work in SA1, proving Deem Britstow was an incedible actor for Eggman. But yeah anyway, revert to the original gameplay with upgrades hidden throughout, but not Tikal and Chaos flashback to a past time crap, I'm pretty sure everyone hated those moments in the game, they were a real drag. And the final boss should be EPIC, close to how Erazor Djinn was in Secret Rings, I must admit his final boss battle was fucking immense, but not EPIC. Perfect Chaos was waaay to easy. Eggman should not be the final boss, but he should be the end boss for each characters' story. The character list should be roughly around the area of Sonic, Tails, Knuckles (his emerald hunting stuff was fun in SA1, but ruined in SA2 by the randomisation), Big, E-102 Gamma, Metal Sonic (the only dark side character since he was truly evil, because Gamma because good), Espio (possibly, that would be pretty awesome) and Super Sonic (no hyper shadow, but I'm cool with Super Knuckles and Super Tails like in SH).
Yeah, that was a big fucking rant/demand/theory thing. I don't usually do that, but you can tell by my name (and most of you know me well anyway) that I KNOW SONIC.