I'm cautiously optimistic. Open World games have a bit of a long history with being giant, open locations with a lot of spectacle but not much actual substance. They tend to be better when the explorable area is smaller, though given Sonic's signature speed that kind of design could be a bit of a problem as the world could end up feeling too small. There's also other open world games with long development times such as Cyberpunk 2077 that ended up releasing in a sub-par state. Additionally, Open World Sonic has never properly been attempted before, so there's no existing blueprint to work off of to make such a thing work.
However, those worries aside I get the impression as though SEGA is trying to push Sonic into a new era and use this game to push Sonic out if his recent mediocrity slump, so I'm somewhat optimistic that they might have some good ideas going on here that could make this idea work. Additionally, I really like the idea of having an open world area in which to search for the Chaos Emeralds in, and I really hope for a playable Super Sonic that can actually fly through the open world in the post game.
If more characters are playable than just Sonic, there's also a lot of potential here for there to be places in the explorable area that can only be accessed using the specific abilities of each character, such as Tails flying up to a high place or Knuckles smashing a boulder blocking a path. The sidequest potential for this is rather enticing, and it would be great if we could get more of a personality spotlight on several characters such as Tails, Knuckles, Amy, Shadow, etc. as well as some nice character interactions. I imagine to switch characters, you'd simply need to go to their "home" in the overworld and speak to them, with unique dialogue depending on who you are currently playing as. Perhaps you could even form Heroes-like teams to switch between on the fly to make use of several abilities at once?
