Honestly, I got the idea of having a multiplier in this while playing Guitar Hero II. It even works in a similar manner. Each ten ships the player shoots, the multiplier increases by one. If the player dies, the multiplier resets. I'm thinking that the multiplier will also multiply the amount of kills needed to reach the next multiplier.
Now I can start designing the first level. I already have all of the enemies for this level programmed, except ofcourse for the boss. I might release a little demo after finishing the first level.