I've decided to play that demo you posted here out of pure curiosity. After playing it I can say that I wasn't impressed with the demo and at times I got rather miffed with what I was playing. Let me try to explain why.
Visually I do think the demo looks pretty nice with smooth animations for Sonic and all the details the sprites have going on, though sometimes I do think the amount of dithering on certain sprites can make them look a bit "grainy" for lack of better words.
The controls themselves are quite decent though at times pretty slippery, though I do wish there were some options to rebind the keyboard keys because I'm personally not a fan of using W,A,S,D to control a 2D platformer.
While the level design is decent for a Classic Sonic game with them having some different paths to take plus a decent variety of gimmicks to interact with, the things that ruined my enjoyment with the levels were the parts that referenced different games like Pac Man, Portal and Puyo Puyo. Speaking of Puyo Puyo, that section in Egg Plant was the most tedious section to go through due to three issues that added up to make it frustrating:
1. You have to put down the puyos falling while at the same time controlling Sonic, which is not easy to do due to you not be able to see the top of the board, and the puyos start falling with gravity as soon as you press down. This makes it really easy to get crushed if you're used to regular Puyo puyo, since holding down usually speeds up the decent of the puyo instead of triggering gravity.
2. As soon as one puyo reaches the top of the screen, no more puyos spawn even if there's clearly enough room for more to fall down.
3. Those freaking bumpers! Seriously why are they here for a Puyo Puyo climbing section? It takes one bump from them to knock you off the tower and force yourself to start all over.
The Pac Man and Portal sections weren't nearly as frustrating with the only issues with them being the Pac Man Robot's tendency of going through any open path it comes across despite me not wanting to go there, and some parts in the Portal section not being very clear for where I could place the portals. Still I think the Pac Man gimmick could have worked better if the maze was a smaller maze instead of it being a full round of Pac Man, and the Portal gimmick could have had some challenges that involved Sonic's physics like what later puzzles in Portal 1 and 2 would do.
So I can see potential for the Pac Man and Portal gimmicks being better stage gimmicks, but the bosses however....oh boy I have tons of problems with these bosses. Let me describe my issues with each of the boss fights:
- Egg Plant's boss suffered from bullets gradually coming out of nowhere as you get closer to defeat Eggman and due to how small the room was, the only way I could defeat Eggman was to spam that "Hard Roll" move until he was defeated.
- Hot Forest's boss was a case of me only being able to stand on the ledge of a platform and hope that either the moving platform in the center doesn't suddenly pick Sonic up or I time my jump correctly so I can hit Eggman without bouncing into the lava. In a game that doesn't have as many invincibility frames compared to other Sonic games, I ended up easily being knocked into the lava and then dying a millisecond later. Then when Eggman was on his last hit one platform was completely covered in fireball spam which is just plain cheap considering the other platform wasn't completely covered in fireballs. If you ask me this boss could have worked better as either a boss in a wide open plain or like the auto-running bosses in Sonic Advance 2 because it would allow more room to dodge the projectiles and still be able to score a hit on Eggman, compared to your boss arena which is just Marble Zone's arena but with a random moving platform added.
- Bright Paradise's boss was the worst one and might just be the worst boss I've encountered in any fan made game in a while. The spike shooters from the top were had to dodge, Sonic is somehow able to jump despite falling in the air, even with the hard roll it was hard to hit Eggman due to his rather small hit box, and lastly
THOSE F**KING BUMPERS! The amount of bumpers constantly showing up to bounce Sonic into Eggman or the spikes, combined with the bumpers that were already hovering around Eggman, those made the boss rage quit worthy. The spike shooters were enough of a hazard to deal with, the added bumpers were unnecessary and only made the boss cheap.
In general the bosses suffered from having very little room to dodge any possible attacks, Eggman's hit box being difficult to make contact with (plus the hit box still being there when he's defeated), and cheap moves being pulled that you won't see coming.
Last major problem I have with the gameplay is the odd lack of polish with the game's engine. You can still get damaged when in the chemical plant pipes, there's no game over system making the lives counter kind of pointless, and I've noticed cases of Sonic not being aligned with the tiles very well.
https://i.gyazo.com/0d19741e1054ea24101cea3f587804b2.png
https://i.gyazo.com/eb2db7e432d53c3c2e4c0b1ba3da2e40.png
https://i.gyazo.com/b588bd1a0b102a909fc4f74da3f8f445.png
This is obviously a demo so these will no doubt be fixed as you continue to improve the engine but I thought it was worth mentioning for the long run. Oh and maybe next time you send out the downloads, maybe don't package both the "win32" and "win64" builds in the same same zip because that just makes the download bigger than it needs to be. It would better if you either did separate downloads for the 32 and 64-bit versions, or just send out the "win32" version since this is just a 2D platformer that isn't quite intensive enough to make a 64-bit version needed.
Overall, I didn't get much enjoyment from playing your demo outside of the extra Warehouse and Green Hill Zone levels which were decent at best, but mainly because of them not having a badly executed gimmick near the end with a cheap boss following afterwards. I can definitely tell you've improved a lot since I played Pepsi in Sonic 1 last year and I do wish you luck with this project, but this game does need a fair bit of work on in order to make it a better experience.
