This might be a completely baseless opinion but, honestly, I feel like Izuka and Kishimoto bringing up Sonic Adventure so much lately just tells me that Adventure-type content is planned to appease fans but not that a full-fledged sequel is planned to release any time soon. In any case, if you stalk my profile, you'll easily see that I'm very fond of the Adventure games so I would love a continuation that picks up the story, tone, gameplay, and music of the Adventure games. I could see many possibilities: it could continue the Sonic/Shadow stages of the Adventure games, it could have Sonic as a sole playable character with sections that offer different gameplay styles (like shooting, emerald-hunting, platforming, or fishing) as seen in '06, or it could have various characters with the same core movement that adapt the classic gameplay into 3D with purpose-built stages for each character. It could blend the abstract aesthetics of the classics with the realistic environments of the Adventure games as seen more in the first Adventure game. There are countless possibilities as long as it adheres to the core Adventure fundamentals (whatever we consider them to be). It's always good to dream.