I recommend (as a gms2 user myself, heres the game i made: https://redfive11.itch.io/round-knights) to add more movement options (such as rocket jumping/double  jumping) and a bigger play area with more platforms to suit the new movement capabilities. Also, you could make the zombies something that fits their movement style, like ghosts or drones or something and add some cooler weapons that have things like explosions, long lasers, and insane knockback/recoil.