4+1/2 stars, this game is amazing but as with any game something can always be improved or added, some ideas I had for a possibility in the next or some future update/episode is:
Ammo Pickups - you can pick up extra clips for your weapon if another of that weapon is on the floor and you walk over it
Grapples/Executions - I've heard this was possibly planned, if so I think it would be a good idea, could let you make some interesting endings to some levels, for example (SPOILER ALER! SPOILER ALERT!) using an execution to kill the large mag agent at the end of episode one
Tricky Campaign - A campaign where you play through as Tricky the Clown, the magic/slowmo bar could be replaced for him with three portions, all equalling different attacks that you'd have to press the shift button in quick succession to perform (example, he could use his speed to instantly arrive in front of whoever the strongest on screen enemy is, second press would be a rapid-fire punch (I think he did something similar to this to Sanford in one of the .5 animations), the third press could be a dropkick/powered punch which knocks the enemy back quite far, it could work on anything other than overly large bosses like the Mag Agent or MK2 GO3LMs.
Jebus/Jesus campaign - A playthrough where you play as Jesus, being able to access his powers such as the bullet catch/deflect shield, the resurrection to zombie allies etc
Custom Character (Arena Character) Campaign - A campaign as your custom Arena character, this campaign would be fairly hard to unlock because you'd have to completely the main character episodes first, and you'd have to have your character upgraded to the max for all his/her skills
Use any character upon episode completion - the name should explain itself, you can use any character from that episode in any of the levels upon the episodes completion (for example, beat 1.5, and be able to use christoff with his halo in the first level of that episode, or use hank in the levels between rescuing sanford and deimos, and the nexus HQ)