Online co-op shmup CounterAttack: Uprising

https://store.steampowered.com/app/451600/CounterAttack_Uprising/

It supports:

• 8 players online (16 with the command /foursquared)
• 4 players locally (8 with "dummy ships" by pressing start with surplus controllers)
• Level Editor (works online)
• Campaign Editor (also works online)
• Steam workshop with a bunch of custom levels+campaigns (some are actually pretty good)
• Massive customization in too many ways to list