I think Fanfics should get a little more creative these days, The type of fanfics I like to read are the ones that are REALLY long stories, at least 100,000 words long or it aint worth it in mah opinion, but thats just me of course I know some people out there prefer to read the short stories that just leave you with that after taste after you read something short but touching but thats not what I'm looking for, what I'm looking for its the LONG stories that take they're time to build up character, scenery, multiple points of view on characters, and good stuff that builds up to a good climax at some point.
Romance is ok as long as you keep the characters IN CHARACTER (I dunno why a lot of people make fluttershy be a misstress...then again...) and don't make the characters fall in love with eachother by just bumping into eachother and making love at first sight.
For example, if you make a Twilight X Big Mac Fanfic don't make Twilight or big mac fall in love in eachother by just looking at eachother or by making them talk with eachother and have them blushing the whole story, Give a Reason WHY they should be in love with eachother, such as Big Mac helping Twilight on something or Twilight Saving Applejack or AppleBloom and then They invite Twilight over to Dinner and they all know eachother and such.
I was never a fan of horror only beacuse I never found them interesting or worth mah time....
Adventure is Something I like the best in mah opinion, It fills in what the original show lacks and lets the writers go nuts on the mysteries and wonders there are outside of Equestria.
Grimdark is what I probably love the most, just like "Adventure" It fills in what the show lacks, the dark stuff that the original show isn't able to show and it gives us the chill of excitement of how our little mane 6 ponies would react and take in the dark stuff in they're lives that they though they would never experience in they're lives.
Oh! and OC's are ok as long as the main character OC isn't mary sue and Kills every bad guy without consequences, give The OC weaknesses and have his moments where it shows that on the inside he is still a pony (of Human if its a HiE story) just like the rest of us