While this is a very fun game, it does get kind of samey after a while. Anybody who's ever played Touhou Project is going to have a pretty easy time with this. Suggestions:
1. More bullet patterns. Currently there are two: aimed and homing. Aimed shots are easy to avoid by streaming. The homing bots are trickier but you can still get them caught pretty easily. (Note: DO NOT CHANGE THE HOMING BOTS. SAID GETTING CAUGHT EASILY IS WHAT BALANCES THEM OUT.)
2. Fragmentation gets REALLY OP after a while.
3. Maybe give the player more shot types? (Never mind I found that)
Really, I recommend looking at the Touhou series for inspiration, it's what a lot of people think of when you say "bullet hell" for a reason.