I am not sure if this thread is old or not but you could add many abilities but I will just be throwing them out there. I do not know much about code so do not go hating on me just because you may think it is not possible.
Ability to fly
Ability to avoid collision with objects
Teleports to/from players (I know this is possible)
Not 100% sure on this but could add players going up to 8 people if you wanted to do Grief
Invisible, what I mean by this is that you do not have to be 100% invisible but you can have it so that either the zombies do not see you or can not find you
Instant Perk
Make it so that the box does not change location when used multiple times
Instead of having weapons names 1-whatever number, maybe actually name the guns, I know you may think that may be hard but you can easily get someone getting the ID's for you by testing it in-game and tell you what weapon is what
Standard Speed and Gravity if you feel like adding it again, such a laugh...
I could probably think of many but for now this is what I can think of. I am just throwing the ideas out there whether you think it is possible or not, feel free to take a look into it, can't wait to see the next trainer you will make!