Isn't writing a joy? It lets us classify ourselves as artists despite how we lack the patience to draw. Or maybe it's just me.
In terms of a story's length, I feel that any experienced writer of anything would tell you not to care; it all depends on the story you're trying to get out. Personally, I'd rather read 2,000 inspired words than 60,000 forced ones. So it kinda falls to the writer to just set themselves down the path and see for themselves where it naturally ends. As long as you're in control of your words, you should know where that point lies, and if you wrap up there, chances are you'll come up with something worth reading. This definitely applies to high-quality horsefics, in my view!
I prefer plenty of dialogue personally, but I've always liked MLP fanfics that sometimes get criticized as being overly-wordy. Dialogue is simply the best way to emphasize that your characters have personalities (assuming you've given them some) and also that they're living creatures with thoughts, not just two-dimensional virtual beings. Basically, as long as the conversations in an mlp fanfic aren't just plot-related, don't serve as padding, aren't dull or pointless and don't either dominate the text or be virtually nonexistent, they should be fine. And critical elements are up in the air, for me. Whatever the writer sees fit works.
The question of FiM content is highly subjective, but personally, the short of it is that I don't like substantial deviations from the canonical material of the original show. That's why I was never a huge fan of the Fallout Equestria sub-universe, on account of the content (100% opinion). Why even make the characters ponies and utilize the world they live in if absolutely everything about them is so un-ponylike? I can see why putting them in graphic situations and making them say graphic things is metaphorically and emotionally powerful, but I have too much respect for the idea of a world where the values of innocence, benevolence and universal good will are upheld to condone fanfics that do an exceptionally good job of corrupting it all for the sake of gratification. But hey, readers should read what makes them happy. And no-one has the justification to stop that, or try.
For adventure, I guess just make sure that your story has a consistent tone. I've read a few great adventure fics that began strongly but ended up feeling thin and stretched as the writer ran out of ideas. So if you've finished exploring the mystical chimera temple and feel like a break from action, pad it with something interesting, like a hot springs "episode" or a sidekick going off on their own minor adventure so that interesting stuff continually happens, and your readers don't notice a quality dip. Sharp eyes, they have.
And for slice of life, you've got to really let your characters speak for themselves. Because there aren't any pony apocalypses or dragon invasions to distract readers, they'll be focusing on your characters 110%. So I like to think out every personality in advance and figure out small things like how to make them talk differently and react differently to things; it's this web of complex relationships that gives slice of life its wonderful flavour. You can do this as you go or prior, whatever works.
As far as experimental narration goes, SeraphStar said it nicely. If you're feeling worried about it though, maybe research some unorthodox methods and use something that another author already came up with for your fanfic. Just for the reassurance of not being the first to try something!
So, hope my "tips" weren't too obvious and best of luck with your writing!