I spent some time to rethink the gameplay and here are my ideas:

• player gets experience points for killing enemies. Higher combo - more XP.
• advancing a level lets the player choose one of three randomly selected upgrades.
• killing large enemies and making long combo chains yields some kind of trinkets.
• the player will use the trinkets to unlock new upgrades (which will be available to choose in further plays).

I started implementing the XP system: [img=https://s3-eu-west-1.amazonaws.com/hplovecar/xp01.gif][/img] I'm still not sure what to do with new weapons - I don't think that putting them in that random-3 is a good idea. Right now I think about an in-game shop, where you can buy another weapon using the trinkets (instead of using them to unlock new upgrades). Do you have any other ideas?