fun. kind of hard on general. some suggestions...
storyline
cutscenes
a page or two on each shooter (bosses not included)
different tanks to choose that have stats, like lower health but bigger shield, and some that have drawbacks but start with upgrades. (maybe you can even have a base with very low health but dual shields!)
info/tips
 "Shield Defense"){kind=icon}
 "Tactics 100 Live"){kind=icon}
 "Johnny Rocketfingers 2"){kind=icon}