Take my advice. Make your creation stand on its own merits instead of trying to 'mine' a fanbase, especially such a miniscule one that Metal Fatigue has.
Attaching it to the game will put certain expectations on your mod, and you'll be heavily criticised for anything which doesn't meet the fan's expectations.
First off, I agree 100% with this. Using somebody's IP like that is very, very risky.
If you make a crap game that doesn't look anything like what you're drawing on, the fans will hate on you, and you'll go down the tubes. If you make a brilliant game that looks like the original, or better, you will get shut down.
There are a few companies that have posted specific exemptions (LucasArts, Ltd., for example)... but for the most part, you are in mortal peril when you try attaching other people's names to your game. I really wouldn't do it, and I'd be very careful about it.
I need a story writer!

Or I don't need one for now!?!?
Definitely the latter! If you don't know how to animate, write CEG code, uvmap / skin, etc., etc., etc. yet, you are a long, long way from having this done.
The very
last thing I'd worry about is your backstory- just write a one-line idea, like "this is a game about two kingdoms, who fight wars with their magical creatures" and mainly just try to keep your art consistent with that goal. P.U.R.E.'s backstory evolved quite a bit, as the game progressed to completion. The main story was in my head about December, but at the beginning, if I may be perfectly honest, I just had a one-liner- "Humanity vs. Evil Robots, and make it not just a ripoff of the Terminator movies". That was pretty much all that I did with the plot, for months!
Sure, sure, I could have written some novel, gotten a million concept artists, and determined the whole thing ahead of time, like the pros do... but meh, it's so totally un-necessary, imo.
Concentrate on building the best game you can... make it all make sense later. People who put the story ahead of the game almost always seem to fail miserably at actually finishing their game, in my experience. So, don't stress about it.
Maybe magical monkeys build magical tanks with magical powers.
Maybe this depicts the ancient wars between the hyper-advanced Atlanteans and the alien-inspired empire of the Maya, using steam-powered robots and tanks that look like metallic dragons...
It matters not. Write something very short, and simple, about what feel you want the art to achieve, and what the game is about, as a game... and just keep it to yourself for a awhile. When you have some great art and a definite style, if you're not a good writer, then find one at that point, give them the simple version and show them the art, and let them do their thing. Mainly... don't get all stressed by this,
it can wait.