Then why make 'Sonic Adventure 3' at all? What DO they want from it?
Sonic Adventure 3 can mean a range of things, I mean look at how much difference there is between Adventure 1 and Adventure 2.
In terms of tone, even though both indicated massive genocide, Adventure 1 tried to skim around the sides of things that did occur whilst still retaining its family friendly tone, Adventure 2 did not.
Sonic Adventure came with a hub world, this idea was expanded in Unleashed but I wish they could go one further. Put Sonic's friends as some of the hub characters, Choatix crew gives Sonic missions, buy stuff from Cream and her mother, Blaze can give you hints etc. It's not essential but I do think a hub world can add to the experience just depends how it's done. But this was something dropped entirely in Adventure 2.
Even the music is totally different between the two. Really it depends on what Sonic Adventure that people wants, I don't want us to get the bad things from Adventure 1 (ie Big, Amy etc) but there are some good things in Adventure 1 that they could expand on further.