MC are you on fanfiction.net?
Anyway you haven't mentioned one way or teh other if you are trying to do soemthing "commercial".
If you're liek me you can write anything you want and there's JACK anyone can do about it, because it for FREE and I am not harming any franchise. In the decades of fanfictio before us I have never heard of a STORY getting a C&D letter...

And trust me if any story neeeded to eb banned it was that
snuff/torture Pink Power ranger fic. Just 2-3 chapters of that was more than I could stand, that guy is SICK!!!

But since you keep asking about "the line" here is it:
In a court of law if the plaintiff show a more than a 30% similarity to their work, they can win a judgment against you for THEFT OF IP. So if you are even thinking of something commercial I'm asking you right now to STOP and work on an
original concept.
If your intentions are the opposite then my apologies for jumping the gun...
Now "independant parallel development is a proven
fact (ie the telephone was simultaneously invented near the same time on the other end of the Earth and one guy was faster to patent it so we remember HIS name (bell) and forget the other guy...
But in this case you already
KNOW about FS2 already and talk of meshing elements of it into your works...