I think you could make the game without powerups slightly more interesting. when you get into a shooting match with another player, it seems like it is just a matter of hammering the gun button. Maybe I am missing something here though, but it felt like there wasn't much evasion built into the game. The boost powerup provides a bit of this evasion feeling, in addition to the fact that it is really nice to just boost around the board, and pop off little shots.
Yeah, I think the lack of evasion (or at least being able to defend yourself somehow) is a bit of a problem. And it gets particularly bad when things get really bullet hell-y. I've been trying to think of ways to fix this, but it may require some more fundamental changes to the game than what I've tried so far.
The behavior of the powerups was a bit unclear. What is a bee mine?

Most of the mine pickups affect what happens when you
collide with a mine. The bee mine is pretty obvious if you do that. Also, some of these mine pickups can be advantageous, so in some circumstances you might want to collide with mines and take some damage in exchange for what they add to the playfield.
Also the fact that the powerups seem to stack infinitely, there never seems like a reason to not go for a powerup. What if you could only hold the last 2 powerups on the stack. Again, I maybe missing something here, and that is already the behavior, in which case maybe that should just be explained visually. I guess there are some situations in which your tank becomes so powerful that you just kill yourself.
Yeah, the pickups are supposed to stack infinitely (or at least until you've exhausted all the pickups in the game). And I think that's part of the problem with the game. I'm still trying to figure out the best way around the whole so-many-modifiers-that-suicide-is-inevitable issue, because I really like the chaos all those interactions bring. It's just that survival comes down to a roll of the dice after a certain point.
I think I might experiment with limiting the number of active pickups, see how that works.
Have you played inside a star filled sky? it really pushes the decisions about what powerups you need to grab, while also having a visual language that shows what the powerups do.
I've not actually. I'll have to check it out. Though I'm a bit apprehensive about the idea of drawing up icons for all 36 (I'm aiming for 64, I think) modifiers that are currently in the game.
Anyway,
here's my WIP build (Windows). In this one you get healed 20% whenever you grab a pickup, and it adds Turn-based mode, zombie shots, mine shots, kamikaze tanks, and camouflage mines modifiers. Not sure how well turn-based mode works...