I think we need a brilliance fridge page, because a lot of the references made only make sense if you have a back ground in Sonic. For example, at the BEGINNING of the game (not much of a spoiler then) Sonic notes that the Green Hill zone look formilar, but Tails doesn't remember ever seeing it. That is because he WASN'T IN THE FIST GAME.

That's how the game is, each era(AKA Generation, duh) have 3 Zones, 1 Rival Battle and 1 Main Boss.

Sonic Heroes was pretty much a direct sequel to the Dreamcast titles. If the Dreamcast didn't flop, it probably would've appeared on it instead. They're directly related. I'd like to note that the Dreamcast was on nearly equal power to the GCN / PS 2 / Xbox.

Now why is there no Shadow The Hedgehog stages, I wonder... but anyway.

Simple: it's Shadow's game, not Sonic's.

I approached this same theory on the Dreamcast trailer on Youtube. Shadow's game was more on Shadow's focus even though Sonic was in it. If Sonic was somehow the main character of that game, it probably would've worked its way into Generations as a main stage.

But then there's the fact that the 3DS version has an Adventure 2 stage that Sonic never officially goes through. Again, Shadow focus for that. I'm probably thinking about it too much, but Shadow's gameplay in that game is the same as Sonic's, so it probably wouldn't matter at all.

Overall, if Shadow's game had a stage at all, it would still break the pattern of having 3 main stages per era. Right now I'm seeing that happening 100% after Crisis City gets officially confirmed.

Also, Shadow's game had stages that were also featured in Adventure 2, so, they could possibly consider it too similar to Adventure 2 to include.

I was actually joking about Shadow The Hedgehog. I just love the game.

And it was fine with what it chose.

Anyway, I still go with my Dreamcast theory. It actually makes a lot of sense overall. To be fair, they couldn't have included Silver without Sonic The Hedgehog 2006. And Shadow already has his appearance via Sonic Adventure 2. It's not an actual problem.

But Chris Highwind has a good point. Many of the stages were references to Adventure 2. Now, one could question why the handhelds got no notes.(outside of Blaze, arguably)

The real reason no levels from Shadow's game appeared in Sonic Generations? It wasn't one of EGGMAN'S Failures. Eggman was a boss, yes, but he wasn't THE boss, or responsible for the boss. Eggman's plan for the Time Eater was to go back and undo all HIS failures.