This is an awesome game! yes I admit it's quite similar to Yu-Gi-Oh, but I like Yu-Gi-Oh, so it's all good. A pretty great game overall.
I do have some suggestions tho:
1. Make a sequeal, or least an update.
2. Some more monster cards. I felt the moster area was a little too limited, branch out to more realistic species, mythical creatures and sub-human characters.
3. More 'Combine' friendly characters and cards. That was one of my favourite parts of this game. I based my deck around the 4 combinations. Add more characters that would support this feature, as well as more to go with currents cards and the current Combine friendly cards. (i.e. Add a bat character to combine with your already used Vampire to make a Vampire Bat etc) Just use your imagination, I enjoyed you "Ninja Rat" quite funny. Unfortunately, I was only able to summon the Dragon Rider and Nija Rat "super mosters," as you called them, during game situations but I would love to see more.
3. Sounds and music feature. A bit quiet, add some bg music and some attack and defence sounds possibly. I would love some battle cries, that'd be awesome.
4. Field Cards. Maybe add this feature to help out characters that are of the same type. A 'Sky' field card helps out flying characters such as the Dragon, and a 'Dark' would help the Vampire and Cerberus.
5. Type Decks. Suggested by others, but supported by me :) I personally liked to choose my decks, but this feature benefits beginners and those lazy people.
6. Online multiplayer. A LONG stretch I admit, but a great feature if you could somehow work it out. A ranking system and everything, it'd be fun!
That is all I can think of right now. Don't take me the wrong way though. I LOVED this game, perfect in it's simplicty too. Take my advice as CONTRUCTIVE critisism, if you plan on expanding this series, keep them in mind ;) I hope you do, I would hate to see this great idea die.
I'm adding this to my favourite submissions, congrats! And I'm voting 5/5!