I'm going straight to business here to gush about something in this game I do not want lost during the "refinement" process.
That being the challenge that is posed of navigating loop-de-loops, and pseudo moebius strips, the fact that turning too fast either via assist of the manual camera turning mechanics, or what I simply feel is inferior during the "free cam", impacts your speed. This level of control, and juggling with physics really made it fun to fail the complicated loop-de-loops or pseudo-moebius strips without abusing the peelout or spindash until perfection was reached, something that can't be said of any 3d game's spectacle events. It really reminds me of those rough unrefined edges the first Sonic game had that helped it stand apart, the idea that in the "fast" stages conservation of momentum was a must, because gigantic slippery parabolas, loop-de-loops and even a few simple slopes were a genuine challenge to navigate or in some cases circumvent all together. Unfortunately I've read rumors in multiple places about you with Murasaki et. al. trying to see if there's a way to further automate the traversal of loop-de-loops plus pseudo moebius strips, fine automate them as you will, platformers are unfortunately supposed to be mostly skill floor friendly.
But to the more, let's put it kindly, "mechanics obsessed" freaks like me out there make the automation less significant in a "hard" mode difficulty, or maybe call it a "hedgehog physics" mode instead, but just allow a mode to keep it as close as possible to the borderline perfection I found in that demo I still "obsessively" play 1-2 hours a day still. Of course I realize the demo has bugs (flapping your wings to glide, kek) in it so refinement will probably see not just the bugs gone, but the quirks some of us may have grown to love, that's fine, but just keep in mind some of us actually prefer the lack of automation found in the demo, some of us unironically enjoyed the challenge. Substance over spectacle, the substance being mastery of how to incorporate the spectacle into the traversal of the terrain.
A critique and PRAISE about your stance of "lives" being a derelict of the arcades in your 2016/17 SAGE interview about S:Utopia and Sonic's game design in general
In an age where videogames have turned into boring interactive movies, or a boring checkpoints abusing snooze fest, I unironically miss the idea of a lives system. Sorry for getting slightly off tangent here, but you can't imagine my surprise then when Doom Eternal added the "no more lives" mode and then made it so all the "lives" you find exploring inside a level actually matters. Back on tangent, that is why a lives system for more "traditional" gamers matters, it incentivizes rationing of resources, makes us reconsider our approaches to levels and incentivizes exploration. In games with scoring systems it also incentivizes whether we play more stylishly to gain points, or play more conservatively to eek out a victory. So to see you try to chuck away a lives system as "just a derelict" in videogames is really disheartening.
However I must give you praise where it's due, we've been long overdue for something more meaningful to be done with a lives system, there's potential for it to mean something more than just a token that says you can continue playing the game and a token you're rewarded for exploration. Most specially in platformers where having collected lives means both exploration and collected more than a certain preset number of collectibles. Excuse me for getting "Satanic" or "edgy" but maybe have a mechanic where the player can sacrifice a life tokens to gain extra speed or jumping power for a limited amount of time, or maybe he can chuck a life token as a CRT monitor at enemies, hahahahaha.
Or hell maybe just add a "no more hedgehogs" mode into Utopia so gamers who don't like lives systems can game and abuse checkpoints as much as they want, while those of us daredevils can farm as many lives as we want and lose more than one in the process of hunting said life tokens.
Additionally I'd like to mention something from S3&K that added replayability even when you played as Sonic that all the 3d entries and even unfortunately to a certain extent SRB2 has neglected significantly.
Those are Elemental Bubble Shields, specifically the abilities and immunities the shields give you. From the water shield's bounce ability plus reflection of projectiles as well as being able to breathe underwater, to the lightning shield's double jumping ability, immunity to electrified objects plus ring collecting abilities, to the fire shield's immunity to fire and lava plus sweet homing attack. All of these added so much replayability for Sonic in S3&K despite his lack of wall climbing, gliding or flying, there's no better thrill for me than edging or sometimes even getting inside of Tails oriented segments of stages in S3&K. Moreover mastering each ESB's general gimmicks plus utilities was always damn fun as well. The fact that these gimmick power ups are now gone, in favor of a homing dash which has heavy auto aim in the official SEGA Sonic games is just anguish for me.
With the above said, I do like how you've managed the homing dash in Utopia, it's not game breaking like SRB2's thok mechanic which gives you speed boosts on demand, nor is it as easy to abuse against enemy mobs like it is in the official games. If you were to add a pseudo EBS abilitty maybe add a shield that gives sonic a dash manuever that sacrifices the current homing dash's auto aim ability for extremely long horizontal jumps. Just a thought.
Also you may have changed stance on this decision, but if you didn't I agree whole heartedly.
This decision being of adding more playable characters, I completely agree on your rants of understanding Sonic's core 3d game mechanics properly in terms of physics engine and level design, before EVEN ATTEMPTING to add different playable characters who will all require different level design philosophies as well.
Finally a concession point in regards to elemental bubble shields;
Yes EBS were introduced in SONIC 3, the third entry into the series, an entry where all of ST Japan plus America's skill and experience with the franchise culminated into all the brilliant, and sometimes inane gimmicks (fuck barrels in carnival) that make the entry so unique to the rest of the series in terms of mechanics. So no, I don't expect you to add EBS into Utopia, nor would I hold it against you if you didn't. Merely what I want to do is remind you of their existence and to you to incorporate the philosophy behind them into whatever other video game design endeavors you may engage in the future.