The experience system is similar to the experience system in my escape mission for my Gravija game. I have one suggestion: increase the amount of experience needed every time the player progresses in a level. Also, I was thinking that each world could be initially tougher too. Since you have 3, you could make one a novice, the second an intermediate, and the third an expert level. The only thing you should have to do is buff up the enemies' stats, and maybe change around the statistics on other things such as the ship's damager resistance, reduce the chance of getting power-ups and such. I've been full of ideas ever since I started playing Devil May Cry 4 lately... The different difficulties does add to the replay value!
I've got a few other ideas if you are interested, and wouldn't mind helping out with some of the programming if needed. I'll try this game out when I get home fa sho!
