I love Keanu Reeves, but I'm apprehensive about him playing Shadow. In the brief clip we heard him speak in the trailer, I wasn't convinced it was a good voice fit. But then again, I thought Idris Elba wouldn't do Knuckles justice and I thoroughly enjoyed Knuckles in the Sonic 2 film. So I'm going to reserve judgement for the final film.
I'm not expecting Sonic 3 to be a great parallel to the Sonic Adventure 2 game. I mean, the Sonic 2 film was basically Sonic 3/Sonic & Knuckles and they barely touched on elements from those games. But as long as they do Shadow's backstory justice, I'll be happy.
Gerald Robotnik was a serious, angry man, so I'm a bit nervous about Jim Carrey playing him. From his one scene in the trailer, it looked like he was Jim-Carreyifying the character; he seemed more like a lovable goofball than the dangerous man locked up by GUN. I hope he at least gets some serious scenes. Jim Carrey loves drama acting and he can be a pretty good serious person when he wants to be. But... this is also a family film, so I think they're more likely to go the goofy route.
I said this about Sonic 2, but I really hope they introduce Amy Rose in this film. Or Metal Sonic. I'd love to see either one in the live action films. But I wouldn't be surprised if they hold onto them for a future release. Can't play all your cards up front.
I also hope Jim Carrey stays with the franchise. He doesn't like doing sequels because, once he's nailed a character, he feels like doing them again in another film is just parodying his own creation. But he's been excited to come back as Robotnik because he claims there are more ways he can grow and develop the character over time. It's not just repeating the same character; we get to watch his slow descent into madness, from the prim and proper intellectual to the fat, bald, lunatic genius.
I'm also worried about Jim Carrey leaving because Sonic doesn't really have any other villains (excluding the comic series). It's always been Robotnik/Eggman at the heart of every story. So if we lose Jim Carrey, the live action film franchise will kinda die out. He's been talking about retiring from acting for a while now, so it will be sad to see him go while the live action Sonic films are doing so well. Hopefully his kids will convince him to stay on and make a few more Sonic films. He's said he only took the role initially because his kids begged him to.