I haven't read this *entire* thread, but I did read the last 20 pages and didn't see these suggestions brought up, so here are my thoughts.
More Safety Nets / Replace Most Bottomless Pits with Spike Pits
If at all possible I would like to request that the devs consider adding a spike floor or at least more safety-net platforms to bottomless pit sequences in Arid Canyon, Red Volcano, and Egg Rock.
I can understand the reasoning for why one could disagree with my idea might be that, you want the stakes to be raised in the later levels. You can't allow the player to be "bumper bowling" the entire campaign; if they aim poorly, they deserve to go in the gutter.
It's not that there can be NO bottomless pits, I just think there are too many in the later stages. I think the stages would not be any less satisfying to complete or master if many of the bottomless pits were replaced instead with spike pits that could allow for recovery. The player is still punished for their imprecision, but there is more encouragement to try again and get it right, rather than getting frustrated and giving up.
Also, I think that using bottomless pits less frequently then heightens the drama of the times when they are used... certainly the pulley jumping sequence in Arid Canyon benefits from a bottomless pit.
But with Red Volcano, there's already a lot of hazards to contend with; having the pteradactyl toss the player in lava, and possibly recovering, is still a good punishment, but it's not as frustrating as being caught and then tossed to a bottomless pit.
I know this kind of thing is so much easier said than done, but I bring it up because I feel it doesn't fit with the ethos of the classic Sonic games in the way the rest of the game totally nails the spirit. For example, Sonic 2, really it's only Wing Fortress and the one pit in Mystic Cave. There are a few odd pits here and there otherwise, but it's not central to the platforming challenge in 2D Sonic like it is in Mario.
The reliance of bottomless pits in level design is something that annoyed me in the Dimps Sonic Advance/Rush series, and especially in the Adventure/Modern games. I feel that part of the core Sonic gameplay, in both 2D and especially 3D, needs to involve a certain trade-off to allow for the mixture of speed & platforming to be satisfying and not infuriating. It's a fine line!
Techno Hill exterior wall color
This level is pretty cool looking in the factory, with the silver/purple contrast, and then seeing the red Egg Corp building against the orange/blue city skyline.
But the initial impression of Techno Hill is off-putting, aesthetically, and I think really it's just the orange/burgundy/brick color in the exterior sections with the gray grass.
Changing that color to something that pleases the eye more, while still communicating the same dystopian effect, would really elevate the level's look.
More pink, maybe? Neon toxic green? Baby blue? Mustard yellow? Maybe just blue/yellow/silver together like Chemical Plant?
A big downward slope slide for Techno Hill Act 2
Speaking of Chemical Plant, I love the high speed first sections of TH1 and certain parts of inside the factory in TH2, but I feel like there's a set-piece room that is missing.
The same kind of slope as in Greenflower Act 2, but... more. Something that evokes the feeling of that slope in Chemical Plant.
It could maybe replace the springs in that room with the conveyer belts where you need to drop deep in the goo to rise up to the other side and take the platform up the goo-waterfall.
Basically it'd be a gigantic slide, somehow you arrive at the top of the slope, then you run or spin down what seems like an impossible height, and then have a ramp at the end to pop up to potentially the max height, and then land down in the goo and under the thing and up the goo-waterfall.
MORE Credits!
I'll go against the grain of the last few posts, be a bit rebellious. The credits are beautiful! Not just the character artwork passing by - though they are wonderful - the sheer amount of people, the mixture of usernames and real names. I actually would not mind more specific credits, like, level by level, song by song.
I guess because it's a non-profit game, I'm sort of automatically more impressed and marveled by the credits - I'd actually encourage the devs to take more bows! I'm serious. Why not? Share a tweet or write a short message.
Could it have a skip option? Maybe, I don't know. Part of me thinks it shouldn't, haha. Just as long as the game saves prior to the credit roll, so you can escape and force-quit the game, but you get the guilt-trip quit messages like - "Please my lord. We worked on this for no reward other than your pleasure. Won't you learn our names?" What's the longest amount of time a credits scene can go before someone turns the game off? Push it right up to the limit. The player should take the time to not only appreciate the credits, but engage in some self-reflection and soul searching, so the longer the better. I'm just kidding.
BUT — what I really mean by "more credits" is a credits/ending sequence.
In Sonic 1, it's a faked highlight reel of your adventure through the zones and then a party in Green Hill. In Sonic 2, it's the comic panels and Tails plane catch. S3K is the plane or Angel Island, but basically a blue sky and cumulus clouds. For SRB2, I think it'd be a mix of still images / light animation like the intro, mixed with "highlight reel" footage, and then some recorded gameplay footage while the name credits roll by on another part of the screen.
So what is the ending sequence of SRB2? If it's not already mapped out, I think it works as a combination of the endings - S3K, S2, S1 - in reverse.
The player needs to escape the space station, presumably on the same rocket they arrived on, or an escape pod.
As the pod rockets down from space through the blue skies, we'd see flashbacks of the player's "highlight reel" footage through the adventure.
At the end, we see the player return to Greenflower, now peaceful and flowing with animal friends, to join a party with the other characters who you've cleared. So if your first clear is Sonic & Tails, they do celebration animations by themselves with the animal friends bopping around. But then clear as Knuckles, Sonic and Tails are already there too. And so on when you complete as Amy, etc.
Whoever is last to arrive - Fang (talks with Knuckles), Amy (Sonic), or Metal (Tails)- once all characters clear, the final ending comes post-credits: Eggman attempts to join the party in his eggmobile. The characters all wack him together at once, sending him flying off into the distance... and destroying his face off of the Mountain.
Take care of yourself
My last suggestion is just that I hope you are doing well and thanks for reading this post that I'm typing and now posting!