Maybe go look at actual successful twin stick shooters. For instance playing that made me want to cleanse my palate so I pulled up Assault Android Cactus, which

Maybe go look at actual successful twin stick shooters. For instance playing that made me want to cleanse my palate so I pulled up Assault Android Cactus, which is everything this is not: it's a super cute game with local co-op, multiple characters with very different play styles, and a nice little selection of power ups. Make some bots that run around in obviously non-tank skins. Weak but numerous, clearly different. Throw in some bosses that require multiple players to destroy. Those two things would make it more amusing when it's empty, AND have a reason to bring friends in. (Name the bosses, keep them down to only a few on the map at a time, if even more than one, keep track of who kills them, hand out medals for doing so) Right now it's way too slow for this but some kind of Chain Combo mechanism always helps shooters have depth - once you have the basics down you can start chasing high chain, maybe have score multipliers, maybe start showing a visual effect for people with a chain going and offer bonuses for taking them down. (In it's simplest form, a chain combo usually starts happening when you quickly kill several enemies without taking damage; every time you kill an enemy start a timer counting down for a second or three, and if it hits zero then you lose your combo. Play a sad noise when this happens.)