Okay. Wow. This thread is quite volatile (as a whole, might I add, so as not to implicate any particular parties in its volatility). I'll preface this by saying that, of the five completed pseudo routes that Mirage mentioned, I was one of their authors. Granted, my pseudo route was of the OC x OC persuasion, but it is no less relevant to the topic at hand. The most noteworthy reason for pointing this out, however, is that, when I set out to write
Greater than the Sum, I had previously published exactly zero works on this forum, much like yourself. And, having been through that experience, I will strongly advise you - and for that matter, anyone without previous writing experience - against writing a route at this time.
Allow me to share a little backstory. GttS was borne of a post I made in an old "create your own character" thread on this forum. The character I made was loosely (well, less loosely as I care to admit) based on myself, which, in and of itself, is a bad idea. However, for the purpose of brevity, I'll not go into the failings of character self-insertion. Having created this "character", ideas began to form about those with whom he might interact. Within a few hours I had finished my first chapter, and within a few days I had finished several more. What I had was messy, poorly written and ultimately ill-concieved.
I took some time to plan out where I was going to go with my fic. In depth notes were written; who, what, when, where, why. I still have that document, and it stands at around a thousand words; almost half the length of my chapters, on average. That planning gave me a solid structure for each and every one of the chapters that followed, resulting in (for the most part) a cohesive story. That being said, I will go on record here to say that Greater than the Sum is not a good story. It reeks of amateurish writing, planning and storytelling. Its existance reminds me of just how bad an idea writing a route was. It is also why, for the better part of the year and a half since, I have written mostly oneshots.
Doing so has allowed me to learn much about writing. There are the obvious things such as punctuation, spelling and grammar, as well as the less obvious things such as characterisation, narrator's voice, foreshadowing, and so on. Constant experimentation in shorter pieces of a variety of natures has allowed me to develop - if you'll excuse the narcissism - a very high standard of writing. I am 110% certain that, if I were to write GttS now, it would inevitably be a much better-written piece of literature on any and all metrics against which you might measure it.
I did not have the opportunity to see what you had written, but going only by the responses in the this thread and your replies accordingly, I'm afraid it does not bode well for your level of technical skill. Please understand that I do not mean for this to come across as offensive. We have all been at that point of amateurishness at one time or another, even those whom you hold in such high regard. The only way that they - or anyone, for that matter - can grow beyond that point is by practicing. A lot. And the best way to do that is to start small. You can bet your ass that DaVinci didn't wake up one morning and say "I'm going to paint the Mona Lisa"; no, he started long before then, scribbling in a notebook perhaps, until he had developed the skills for which he is reknowned to this day.
I'll not comment too strongly on your premise. I believe it detrimental to limit oneself to writing only certain stories if the alternative might not share the same themes or might require an asspull or fifty. You are free to write whatever you see fit, provided it has, at the very least, a tenuous connection to the source material. What that could entail is entirely up to you; however, please understand that the further you stray, the less likely it is that you will attract an audience, regardless of how well-written your story is.
Furthermore, perhaps the most important thing to consider when writing is how to deal with criticism. Often, for first-time writers, a lot of criticism is received. Likewise, it is often the case that first-time writers will reject this criticism, whether it be constructive, or harsh, or whatever else. The worst thing you can do - even if you might feel like you're being criticised unfairly or even bullied - is to become offended. These people are only trying to help you improve, as much as it might seem like they're discouraging you. You must discard your pride and put your nose to the grindstone. It will take a lot of hard work, and sometimes you might even want to quit, but those authors you admire did not get to where they are without hard work. If you want to be like them, then you must work as hard - if not harder - than they did.
It will be tough, and if you do, in fact, speak English as a second language, then it will be all the more tough. But that will only make your success all the more meaningful when you do, indeed, succeed. Do not give up. Put what you have so far on hold. Store it in some document elsewhere on your computer. And go about writing as much as possible until such time that you possess the skill level that you see in your favourite works. Then, and only then, should consider writing your route.
Good luck, and godspeed.