I remember plaything this earlier and really enjoying it, and I enjoyed playing it again! That being said, I apologize because I'm about to be a Negative Nancy (rolling with the pun here):
In a stream on Thursday, Mark Brown said that the reason he gave Only One examples in the jam theme video such as your "only one bullet" was that he felt they were the obvious answers to the theme (this was not communicated well in the video). We have seen this in the jam with there being a large number of "only one bullet" games. I'm a big worried that "only one bullet" paints you into a corner when it comes to design options. That being said, your plays the best out of all the ones I've experienced, this jam or otherwise.
To utilize the theme of the jam well (in my eyes) you have to build the game around the consequences of your "only one" mechanic. In your and other games, we have seen the consequences of having only one bullet being:
- I must bounce the bullet in order to hit multiple enemies
- I only have one shot per level
- I have to make sure my bullet does not break
- I have to collect my bullet in order to fire again
- In order to effectively fight enemies, I must use some telekinetic-like power to recall or push or move my bullet
- Since there is always one bullet, I can teleport to it; if you had multiple, you wouldn't be able to tell which you would teleport to (I have only seen this in yours, and this mechanic is what makes yours stand out and be so much fun for me)
You have 4 and part of 5, but it might have been interesting if you also were able to push away the bullet (they do something similar here: https://itch.io/jam/gmtk-2019/rate/462860). Pushing away the bullet also works well with your teleportation since you can push it to a more safe area behind enemy lines. There are probably many other creative consequences of having one bullet that I'm not thinking of that you and other people can and will come up with. I'm not advocating for implementing everything here; I'm just listing what I saw in the hopes it inspires you.
Another thing I've seen people do is design enemies around having one bullet. The only example I can think of right now is an enemy that can only be hit from the back. One thing I don't remember seeing, but probably exists is a special enemy that can break your bullet if you're not careful (this could be frustrating). A variation of this could be an enemy that freezes your bullet or hits it back at you (so it damages you). That could cause one of the gameplay state changes Mark Brown seems to like where you go from being powerful with a bullet to running away.
I hope something in this post is helpful. Again, I want to reiterate this I really enjoyed your game. I'm just being a bit more negative here in the hopes of giving you helpful feedback.