Been playing this awhile now with a few local yokels, and this game...
It's already A+ material. Only thing that seems to feel off is hit-recoil. There's no pushback *at all*, and this is a pretty critical element of a game where the players are fundamentally fighting for "control of screen space." The sound for impact is there, the delicious gore is rewarding, but there is little spacial consequence to scoring a hit.
This kind of thing *can* be countered with a once-a-floor force-push that each hero player can activate, by panic-pressing the attack + special buttons together, in case they get pinned in, maybe a 1-second stun and huge knockback to the monster characters IF they're within a pretty tight radius (15 px diameter, perhaps); in order to counter stunlock. I presume that, primarily, is WHY there isn't tremendous hit reactions.
While the animations in the game are FREAKING SUPERB ACE-LEVEL SHISH; the ambiguity of them and lack of physical feedback makes it retardedly hard to tell when you're hitting something, or where the hitboxes may/may not be. That was the biggest niggle going around my crew. The second was with the monsters in particular - it would be very helpful if there were one noticable feature of each of the creatures that became tinted, depending on which player was controlling it. Controlling the ghost-things and slimes was fine, but when people started controlling creatures en masse, everyone quickly got confused as to what they were.
Regardless of whether you put reactions on the hero character, the monster ones certainly need some, because it goes back to space control - the hero player needs a range to keep monster players in check, the monster players need range to keep the hero player in check; but the hitstun/pushback to monsters is critical to balancing the "many vs. one" approach that the game is built upon.
You can also add some simple, fun variation to the shot traps - some of my favorite variants include burst-shots, bank shots (one bounce) [this would add SO MUCH FUN to USING them], and spread shots.
PS: Your AI/bots are at a *very good* competitive level. Do not let ANYONE fool you into thinking it's too hard or too *insert adjective here* - it's a tiny bit above your core player level, which is GOOD, it pushes players to improve without making it feel impossible to do so. This is how you make players good in solo settings, which is what makes the game SERIOUSLY FUN in SOCIAL ones.
PPS: Super jellybeans of the artstuff; it's the troublesome part of my projects. Your application of stretch/squash, blur, and line of action is almost unparalleled.

Last I saw it applied so well, "Looney Tunes/Merry Melodies" was a thing.
PPPS/EDIT: Oh, and one more potential gameplay element came to mind - PLAYER BAITING. Giving the monster/ghost characters some kind of switch they can press (or not press) to grant the hero player some kind of critical reward, but at the cost of some strategic positioning, or to put them at increased risk of the traps. Not sure on the details; but I thought it would at least be worth mentioning and bringing some attention to.