> Sonic R is 3D rendered but it's a racing game.

Only superficially. It's really more of a racing and platformer hybrid. In fact it's probably got more platformer elements than racing. Though personally I don't think the gameplay works at all.

It's still a 3D Sonic game even though it sucks :)

> Sonic Jam's 3D level is just a level select, not a full game.

I said this myself. However lets not forget that the 3D zone did have have some mini-games in it.

> Sonic 3D Blast is isometric which isn't really it either.

Indeed it's isometric (I also said that myself) but you do have movement in 3D space. It's just not 3rd person. If you want to get pedantic then arguably this fan game isn't 3D either because the Doom engine is regarded as 2.5D.

> Sonic Adventure has lots of different gameplay styles for the different characters, none of which fits classic Sonic (Sonic's levels are too on-rails)

The classic Sonic games had different gameplay styles for different characters. But like with the classic Sonic games, you don't have to play as those characters in Sonic Adventure. You can just play as Sonic if you wanted (unfortunately the same cannot be said for Sonic Adventure 2).

Also the levels are not on-rails. You have full motion in 3D space. The camera adapts to angle it towards the next path (maybe you're confusing "on-rails" with any game this isn't "open world"?) but that was because the game came from an era when controllers only had one analogue stick and game developers were still figuring out how to do 3D on consoles.

> Sonic Generations is extremely on-rails, while the classic Sonic levels are classic Sonic in how they play, but including being 2D gameplay :)

Yeah Generations is on-rails but it's still 3D. To be honest I think this style of 3D suites Sonic better than open world.

> Sonic Robo Blast 2 is the first game I've seen that: 1) has full 3D movement and 2) classic Sonic exploratory level design.

I think you're being too restrictive in your definitions here to suit your conclusion. For example of the games you've dismissed are more 3D than the Doom engine and have more exploratory level design than many of the classic Sonics. Sonic R for example is all about exploration (which, to be honest, is one of it's issues given it's a timed game). Sonic Adventure had Adventure Stages that were all about exploration. And there's a whole plethora of bad 3D Sonic games where the emphasis is on exploration. Then when you look back at the original Sonics, they actually weren't about exploration, they were a path from A->B. Sure you had a few forks in the path but all the forks converged on the end. If you wanted exploration then there were plenty of better suited platformers (like Super Mario Bros) that had more secrets, hidden levels, multiple exits on a level, etc...none of which Sonic had (and I say this as a massive Sonic fanboy!)

What this game does that none of the 3D Sonics have done is gone for the heavily pixelated 16-bit style colour palette. Which is cute but honestly without modern lighting effects (which the Doom engine doesn't have) that actually makes the game harder to play is it's harder to distinguish between platforms in the distance. I don't mean this as a negative against the game, it's intended to be a homage to the 16-bit eta games. I just say this to point out that there is a reason why the 3D Sonics are styled the way they are.