I disagree with Davew. If you keep the gameplay the same, then you should just add more of everything, so as to give the game more variety and appeal. Double the amount of markers, for instance. Add more upgrades available for them. But keep it arcade and not getting to realistic. On the other hand, if you do take Davew's advice, then i think you would need to change the gameplay significantly for it to work, cuz otherwise the game would become too repetive (which it already might be as it is). And it seams that everyone wants the gameplay to stay the same, so therefore you should just add more stuff.