My story has a similar concept to a Kingdom Hearts game?

i say go for it. i am a story writer myself, and have written a series called "One Destiny" that i still need to get published. all of my ideas are original, but i understand sometimes you'll find an idea you have created is similar to that of another story. i also have creatures similar to the "Heartless" in kingdom hearts, but theyre abilities and thinking are wayyy different. if your creatures are similar to the unversed, just make them alittle different, like clever thinking, the ability to wield weapons, or a society of creatures that you've created that are more intelligent than the lesser ones. i'd say you probably have good ideas, and you should keep on with what your imagination brings you to write out. maybe one day we could compare our famous stories in the future. ;) glad to help! good luck.

A lot of things now a days branch off of something else popular, you can't come across anything without some similarities to something else.

I say, go for it, see how it turns out

Many story share similar concepts like this. If that's the only similarity they share, then it's perfectly fine.

Sure, as long as it isn't word for word the writing world is free. You might want to consider fanfiction (for fun) should your writing passion and fandom intertwine.

Do it! I'd like to read it sometime! ^o^