This is a really great idea! The overall presentation is great and the game seems to play pretty solid. I've seen some people mention that Sonic feels weird/slow but I found him to feel pretty good. The music is obviously good being ripped from Sonic3D Blast but it sounded like you may have had some slightly different remixes at certain points? It's been a while since I played the original but if so, very cool.
I do have some criticisms though:
-The special stage is honestly horrible. I would give it a 0/10. It is basically unplayable. Sonic handles HORRIBLY, and the rate a which rings and bombs appear to 'grow' as you approach them feels totally wrong. I would personally recommend completely redoing the special stages and releasing an update with them. Maybe simplify them a bit and make them more functionally/aesthetically similar to the Sonic3D Blast ones (which are already a derivative of the Sonic 2 ones) rather than very poor quality clones of the Sonic 2 ones
- The Ring Item Box sound effect sounds weird, not sure why you changed it from the standard classic one.
- Sonic 3D in 2D is kind of an awkward name, and implies this game would be more of a faithful recreation in 2D rather than a pretty separate idea loosely inspired on Sonic 3D blast. I think not naming it 'Sonic2D Blast' was a missed opportunity :-(
- Obviously it's way too late to make a suggestion like this, but again I feel it was a bit of a missed opportunity to not focus on what really made Sonic 3D blast different than other Sonic games. It wasn't just the isometric 3D perspective.. in fact in other regions it was even titled 'Flickies Island'. It would have been cool to keep the concept of saving the Flickies and bringing them back to the warp ring as a gameplay mechanic.. maybe have more exploration based level design and remove the time limit and make it function almost similar to Toejam and Earl 2 panic on Funkotron where you need to find all the 'earthlings' and deliver them to the end of the level before moving on to the next Act.
Sorry if my criticism comes off harsh but overall I do want to say that you did a really great job and I only bother thinking this much about it and responding because I found your work inspiring! I do hope you continue to improve this project, especially the special stages, and I look forward to any further work you decide to do similar to this.