Some random accumulated thoughts:
- At the moment, it feels like attaining a significant altitude/vertical speed is a liability. You don't accumulate down-boost, you can't collect yeets/sodas/etc, and it's tricky to tell whether or not you're about to crash into that Bae coming up. The only advantage, aside from some one-off altitude achievements, is that you don't need to worry about meatballs or Baes, except the ones you blindly crash into at 25 meters per second.
It's making me start to think that yeet-boosts (and especially Mumei's big toss) aren't really helping. They might even be liabilities. To counteract this, some reward for getting high might be nice. Maybe accumulating XP faster the higher you are? Or some new mechanic, similar to the down-boost, if you want something less simple yet disruptive to existing mechanics. - Speaking of down-boosts...only being able to use them while moving up is a weird restriction. They're sometimes useful for exploiting an opportunity before you fly away, but the narrow angle windows and how quickly the angle bounces around, it's tricky to pull off—you need to hit F at the right moment, then release it at the right moment. And since you can't use it while moving down or sideways, it doesn't help you avoid hazards unless you can predict where you're going to land early in the arc—and if you're moving with speed, you'll probably stop moving up before your landing site comes onscreen.
Down-boosts feel very restrictive. I feel like there's an intended use case I'm missing, because otherwise it feels too restrictive to be useful with any regularity. - The camera feels weird. If I'm reading its behavior correctly, it has two zoom levels it can slide between; this is fine under certain ranges of speed/altitude, but also sometimes feels weird. I assume you tried a more granular camera and something didn't work?
It also feels way too easy to fly off the top of the screen. That feels like it should be exciting and uncommon, but it's common when you combine a decent base speed with a yeet/boost. Under current physics, a 10-meter ceiling feels too low. A fairly mundane 12 or 15 meter apoapsis doesn't feel very different from a 60- or 100-meter one; its just IRyS staying offscreen longer. - Airspace feels underutilized. Flying items are fun but simple, Monopoly boards are fun but random. (Though I like that both good and bad are better/less bad if you're low on boosts. Helps with strategizing.) Both are rare. There should be less stuff in the air than on the ground, but it would be nice if there was more stuff up there. It would complicate flying items, though.
If you take this suggestion into consideration, consider adding some kind of indicator for upcoming airborne items. Actually, maybe consider that anyways—it could help make use of flight-item boosts. - It would be funny if IRyS's voice grew faint at high altitudes. Imagine a "WHYYYyyyyy...?" trailing off as Mumei yeets her into the mesosphere.