Flow, focal point, and blending, those are my main 3 criteria
Your signature should have flow, your eyes should look at the sig, and they should be able to flow over it and focus on one point, the focal point.
Also, blending. Your render and your sig should blend together and look like they are meant to be together.
These are the main also the points you should work on, before you make your sig, you should have an idea of what you want ( you don't need to, but it helps

) I pick out my render with a basic idea of how i want my sig to flow and what colros i want to use. Your sig isn't bad, but you will find as you keep making new ones, that you can improve your old ones in many ways. Just keep making them m8