Single Status Update
I just remember back around 2005/2006 when rumors were floating around about Knuckles, Tails, Eggman and even Silver getting spinoff games down the line just like Shadow did. Even if any of that was true or even pondered by the team in any capacity, its too bad the one-two punch of ShtH and '06 both being terrible/ received poorly shut down any and all ideas of doing anything largescale involving anyone besides Sonic again.
I don't recall such rumours myself. But SEGA did conduct a public survey to determine what characters specific spin offs fans were interested in seeing in 2005. This survey didn't result in Shadow (it was already well into development and was announced very soon after the survey), but they were definitely floating the idea.
I think that following '06, either Nakamura or Iizuka said that '06 was specifically called "Sonic The Hedgehog" and boxart designed in such a way so that they could have a trilogy of games to follow the main Hedgehogs. This is even why they called the new guy "Silver" instead of "Venice" or anything else; they floated the idea of giving him a spin off very early on and wanted a consistent naming convention of "S the Hedgehog". I can try to find the source if you're interested.