i ono gets boaring make more guns but make them different it seems they are all the same but u just call them something else, un upgrade screen could be good, such as an ability to increase clip size(oh and u need to make the clips in this game less) or increased damage etc... and add a difficulty setting all u would need to do for a difficulty setting is change ur health
go to villages first they are the easiest then once you make more money go to caves then after caves ship wrecks then factories and then the mother ship