Like lots of other people, I've been on a Sonic kick after the third movie came out. The movie was the best thing I've watched in a while (I even ended up watching it a second time lol). It was a really good interpretation (probably one of the best) of Shadow's character, and I loved all the homages to Sonic Adventure 2.
Sonic Adventure 2
Speaking of Sonic Adventure 2, I decided to grab it on Steam while it was cheap for the Winter Sale; and I gotta say, it's just as good as I remembered (I last played it a couple years back on the 360). The story isn't my favorite thing ever, but it's good by Sonic game standards, and I love pretty much every stage aside from Mad Space and Security Hall (for some reason I just don't like Rouge's stages). The extra challenges for emblems are a fun way to revisit them too, though I find the 100 Ring missions to be particularly annoying in the non-speed stages. I downloaded a couple mods too; namely the Super Sonic/Shadow mod, HedgePanel for faster somersaults, Sonic New Tricks for better controls, and a couple Chao World mods to give that more depth. So, obviously, I didn't play through the game as it was originally released; but I don't really think that matters much. I'd still say SA2 is up there as one of the best Sonic games. (Sega please make a Chao Garden mobile game or something I beg you)
Sonic Movie 4 Speculation
Warning: There are spoilers ahead for Sonic 3, so maybe don't read this part if you still feel like watching the movie blind.
After watching both post-credits scenes, we obviously have a general idea of what's gonna be in the 4th movie. We know Shadow is alive, though whether he'll play a large role in the story we don't know yet. I really hope they don't pull the "Shadow lost his memory" shit like they did in Heroes, I've never really been a fan of that and it would make the ending of the 3rd movie kinda pointless if all that character development for Shadow just vanished.
We also know that the 4th movie will feature both Amy and Metal Sonic(s?), which leads me to believe that the movie will be at least partially based on Sonic CD; continuing the trend that the 2nd and 3rd movies started (being based on Sonic 3 and Sonic Adventure 2 respectively). How much of CD they'll include is up in the air, since the 3rd movie excluded the Biolizard entirely and changed Shadow's origin (to be an alien instead of an experiment). I have a feeling they won't include time travel, since that would make the movie universe a little convoluted, but I'm not against the idea of Little Planet being incorporated in some way. My biggest worry from the scene is the fact that there are multiple Metal Sonics. I've always loved Metal Sonic as a character, especially for acting as a true rival for Sonic who matches his abilities almost perfectly (as opposed to Shadow, who isn't really on Eggman's side anymore). So if Metal Sonic in the movie universe is just a bunch of canon fodder robots instead of 1 really strong robot, I'll be very dissapointed. But there's no way to tell until the trailers come out I guess.