Awesome! I would love to play it when you publish it! I'm pretty new to making these kind of games tbh but heres a bit of what I've learned so far by playing other games.
- Player inertia: I've noticed that most precision platformers have very low inertia when stopping/changing directions, depending on how slippery you want your game to be, for speed I wanted that the player movement responds exactly to their inputs so they can make quick decisions and to promote precise landings.
- Air control: I think this can be an interesting element to experiment with, how much you want the players to commit to a specific jump?
- Hold jump button: The longer the player presses the jump button the higher the jump, this is very important for the precise feeling.
- Feedback and juice: Every action the player does need some sort of audio visual feedback! make sure your main mechanics are satisfying to execute because that's what the player will be doing most of the time.
- Level mechanics: Try to extract the most play of each mechanics in the levels. In speed I tried to mostly introduce new mechanics on each stage and start mixing them on further stages.
- Watch the difficulty curve: this is something I'm not super happy with Speed, make sure the difficulty goes slowly up, let it peak and then drop it again, with this you can keep an engaging game rhythm.
- Coyote time: You already know this!
This is some of the lessons I learned making Speed, I think these are engine agnostic too. I hope these are helpful, Good luck with your game!