I like a lot this game : original and fun.
Some advices, but i know we're in indie area and it's a lone wolf project,
so there are only ideas

- combos perhaps with some characters ?
- big blood effects ?
- original missions : kill a character boss before he reach a point by walking.
(you would have first to quickly kill the incoming ennemies before)
deliver some bombs to some points and survive arriving ennemies
etc ... play GTA game to be inspired for missions
- incorpore mini games : drive a tank on a straight raod and kill as many cars
as you can , with gold medal a reward could be obtained like some
shield that could only be used during two levels
etc ... etc ..
play GTA or Zelda to be inspired for mini games and make yours

In fact you could incorpore some little RPG things like in Zelda games :
some upgrades in weapons , character or anything you want !
A last point : you must reward the player to keep it playing :
money/souls when killing people to buy devastating powers when a furry bar should be full or to buy any reward you can imagine

just let play your imagination , and you could achieve some
Nintendo entertainment style game

But for now , great original game i like it a lot
