I like how you have a low colour count, using warm tones. The monster colours really stand out, the smaller characters seem lost in their less vibrant darker tones but that's not a crit, I accept it because the image works so well. Nice dynamic composition. Interesting camera point of view. Really nice work.
I wouldn't add a background at this stage (if it were me), unless you really wanted to go back to it. I would just find some excellent grungy texture and fill the negative space with it. I think the gradient works, but a textured effect would be so much easier to do and more interesting. It would certainly suit the style of the image in my opinion.
That's just me, and no doubt everyone else will have a different spin on what works, main thing is that you're happy with it
Kraiger! Thanks man. Eric and you bring up a good point. I've made a version of this for printing which is lighter and there's some more highlights on the three characters and robo. I think It'll look good in print