Ideas..
I have just some ideas, which you could use if you make some repairs or even a new one.
1. Take the invincibility away or make it last less.
2. The bounce from getting hit made me angry because some times i "flied" out of the rooms.
3. You could make some kind of money and shops.(You could buy new weapons,upgrades or something else)
4. Make some kind of adventure. lol