Hi. This game have a lot of potential, but is sorely lacking end-game content. I suggest adding an endless dungeon, where players compete against each other in how far we get within a certain timeframe, for example 24 hours. With a public ranking in-game, and some sort of rewards this would go a long way to add longevity to the game. This would basically give us a PvE competition. Right now, once you finish the campaign there is literaly nothing to do.
Check out Hawk: Freedom Squadron for an example of how they do it (Contest-mode and just recently Assault-mode).