Hey @kimberly, thanks for inviting feedback on how you might turn this into a full game!
First of all I can't help myself commenting what I love about what you've already got. The music is DOPE (seriously I'm tempted to just extract the mp3 and save it), the art style is exactly the kind I prefer, and the game loop is really fun. The upgrades seem well considered and interesting. I noticed a bunch of polish like the subtle light that always points to the rift orb you need to collect. I'm a real sucker for time dilation effects too.
I think to briefly sum up what's missing from the jam version of your game, here's a link to a 44 minute youtube video: [the art of screenshake](https://www.youtube.com/watch?v=AJdEqssNZ-U).
lol. Just kidding. Unless...? Actually I do think it's practically essential that you watch it, because everything it talks about applies to your game as well. Sound effects, recoil, screenshake, give enemies some kind of death effects rather than instantaneously disappearing, etc etc. Each one of those is going to make your game more fun to play, and the sum of doing a lot of them is going to make a gigantic difference. Thinking about it, I'm actually surprised at how fun your game is _despite_ missing so many of those specific details (my best guess is that you have enough of them, like enemies getting knocked back by bullets, but maybe most of all the badass music is doing a whole lot of work here!!). Though I hear you, letting the programmer eat and sleep is important too. :) Communication-wise, it took me maybe two deaths before I figured out what the game actually wanted from me, until then I thought the grayscale/slowdown was just me dying from low HP. Also on communication I didn't read the game description until after I played so I didn't realize I could roar - making that evident in the HUD would have clued me in that I was missing part of the game mechanics (although.. thinking back, I did see an upgrade card for roars but didn't know what to make of it).
For a full game, I think the key is having multiple level layouts -- maybe even double down on that, making it into more of an adventure game, rather than an arena battler? That would get you beyond just "delaying the inevitable". After all, I think probably the most important part of an interesting game is having the player learn some new game mechanics, then challenge them with a bunch of different curated scenarios and variations that each test that they really "get" it. Then repeat. Some other smaller ideas: Have different weapon/bullet effects look visually different, and maybe allow switching between weapons or characters. Similarly have different enemies be more distinct (maybe some enemies explode on death, others hover around the rift orbs, that kind of thing). Boss fights. I feel like the rift could be more intrusive as you get closer to running out of power - maybe touching the edge of the stage immediately kills you, and the rift shrinks (in some sort of interesting way) as you get closer to losing your power. Multiplayer obviously!
The game that this most closely evokes for me is Nuclear Throne, so you might want to check that out both for inspiration, but also to make sure your game is clearly different from it.
Anyway thanks for the really fun game, and the unusual writing prompt! I focused on suggestions and critique since that seems to be what you're looking for... you already know you've got a gem here. :) Best of luck turning this into a real game, be sure to @ me when I can wishlist it on Steam :D