True. But Takeshi Iizuka, designer on Nights Into Dreams and the first Sonic Adventure, said in an interview he didn't want to make "Adventure 3" because he was afraid only hard-core fans of the series would play it. That's one of the reasons why most Sonic games now have a stand alone name.
It's sad to him that Yuji Naka was in charge in the serie instead of him. :/
Since Sonic Heroes follows the Storyline of Sonic Adventure 2 has similar gameplay mechanics (different but still similar) it's safe to say that Heroes is the True Spiritual Successor or the True "Sonic Adventure 3".
I see Sonic Heroes more like a spin-off to the Adventure serie, since it try to connect itself from Adventure 2 (storywise), without really doing it. Shadow the Hedgehog was market as ''THE'' sequel of Adventure 2.....What a mess...













