Awesome job dude. I think this is a nice start to a signature but its just not finished yet. I think you should add more to the left side to get rid of the emptyness. An idea would be maybe start off by adding some space/star stocks and expand on them. (Go to google, bing, yahoo, anywhere and just look up space/stars and you should get some nice images) Another idea is maybe move your focal over just a little bit towards the center (rule of thirds).
As for your other images, I honestly think that those arn't your best works. Out of all your signature, your mario one is definitly your best. In all your other signatures (example, Katy Perry) you use the bubble effects, which is nice, but then the bubbles become overcontrasted and oversharpened. My advice to you would be if you are going to use bubbles, the only change you should make to them is colors ( don't contrast them or sharpen them). Another piece of advice would be to stay away from tutorials that make you sharpen/adjust contrasting for now because when you start to play around with that kind of stuff, it could kill a signature in seconds.
Otherwise, your doing a very nice job so far dude. Keep up the awesome work. If you need help in regards to finding tutorials, let me know because there are many websites out there that have tutorials that you could follow.