I was a huge Sonic fan as a kid -- lost count of how many times I played through Sonic 3 / S&K (and enjoyed the idea of, but got quickly bored with, Sonic Spinball) ... so now that I have young kids of my own, I've turned them into Sonic fans also.
The other day, while working on one of our pins together, my 9-year-old daughter asked me, "Is there a Sonic pinball machine out there, and if not, WHY NOT?!" (It never made sense to me either why Sega never made one.) We chatted about fun ideas for what a Sonic pinball would look like -- including a vertical loop, a bash toy (like AfM) for Robotnik, colored balls for "characters" (Sonic, Tails, Knuckles) and a multiball mode with all three of them out there at once.
On a whim, I decided to search out here on Pinside today to see if there were any rumors / plans for a Sonic machine. And I found your thread.
I called my daughter in here and we walked through your development and videos together; WE ABSOLUTELY LOVE THIS. We love that you had some of the same ideas we had, and we love your integration of even more of the original games' features -- the spin dash, the end of stage gate, the music from S3/S&K. She said "I totally want to buy this game" at least 20 times (and I had to explain that it likely wouldn't be for sale, nor could we afford it if it were, lol).
(Per my daughter's request: she would like you consider colored balls for each character, though you may be too far along and/or don't want to complicate it further. I don't know the mechanics of those games that utilize different balls, e.g., TZ, Avatar. In our fantasy version, the game can either sort the ball(s) and/or tell which ball is in play next and updates your on-screen character.)
We are SO following this thread and are looking forward to seeing further progression. VERY WELL DONE, we applaud your hard work. From these Sonic fans to you, thanks for bringing to life what we were only dreaming about!