You know I actually rather like this, as far as wallpapers go its pretty cool. I can really see that your trying to work a bit more on tags and you are certainly improving! In this one you have a single focal point which is really beneficial.

For a sig though I guess there could be a little more, anime/cartoon renders can be quite hard to work with as the background needs to be in the same style as them (Ie it doesn't look right if theres an anime render and a real-life city scape or something), the best way I find to make a sig of those kind of things is to use smudge.

Once again just work around the render, it doesn't really matter how much you drop into the background as long as theres enough.
I can also see you've had a go with gradient maps - which is fantastic! Maps are great for changing colour or improving depth, the only downside to them is they can be a bit harsh lighting wise. Photo filters also work quite well.

Remember that if you think a sig is coming out bad, sometimes certain renders just don't work - I would say half the skill in making a sig is finding a good render.
Also, focus on the lighting, see where the light is shining on to the render and work a light source into the background, and watch for any areas that look too sharp - ie on this wallpaper you can see that the edges of those bits of fire/shrapnel are quite fine.

I would suggest grabbing a big stash of resources to play with - C4D packs etc...
You can always turn down the opacity on any layers that look too extreme.