The game is pretty fun, but ring out is too easy and I really wish there were more options for looking around. Not against the multiplayer option, that would be a fustrating game for sure.
I agreed that a shop, experience, or upgrade system would not fit with this game. Perhaps some fleet selection would though. Before battle you might be give some choice in planes, how they are outfitted.
 "Gravitee Wars"){kind=icon}
 "Epic Combo!"){kind=icon}
 "SteamBirds"){kind=icon}
 "Phage Wars 2"){kind=icon}