The game is very stingy when it comes to actually useful upgrades such as projectile speed, air space and amount of projectiles on screen but generously provides useless ones like projectile life time and bounce amount.
So eventually if you didn't pick super speed as an upgrade, you're shit out of luck, cause you'll be spending all the time shooting down enemy fireballs, cause you have only one projectile to go around and it's too slow to reach an enemy before you need to destroy fireballs again.
I'd probably remove projectile life time entirely, so that it lives until it has bounces left and add a possibility to unlock more upgrade slots and re-rolls (e.g. you could add special enemies, that disappear after a set amount of time, but if the player manages to destroy them, he gets a re-roll).