Might as well get this out of the way. I'm presuming that the engine is in no way set up for multiplayer? I wouldn't normally ask, but in the past Arcen have shown quite a lot of interest in co-op and multiplayer. I believe that there was actually fairly serious discussion about a possible multiplayer implementation for Bionic Dues, even, and the problems were more mechanical than technical. Mechanically, this game seems like the most suited Arcen game for multiplayer since the Valley series -- probably even more suited than those, actually. I'm thinking that co-op would be 2 of you entering a floor and you can split up or stay together to clear it. Both players must be present to warp to the boss, blah blah blah.
Commercially, of course, it'd probably add a lot of value to the game and the SHMUPPY gameplay would surely suit a co-op implementation. If it's completely out of the scope of the project for now (which it seems to be), then how about for future DLC content (assuming the game sells a bucketload, of course)?