It would be great of course, but it would be a tall task for any studio, let alone Sonic Team. My guess is it will be sort of like Sonic Adventure, where the open world is a hub of sorts (with enemies and tasks strewn around) and "missions" that replicate the boost levels.
The issue seems to always come down to creating enough assets for Sonic to blow through while making the game last longer than 3-5 hours, so I would guess an open world could be used to pad out the playtime with backtracking, similar to how Mario 64 got the most out of its level design by asking the player to go through them multiple times, sometimes with minimal changes to the map.