Don't be throwing random stuff in, stuff that doesn't contradict itself. Best, yet most controversial example is the god damn Bible.

But you're probably to lazy to read the Bible (or you're just a different religion, I don't got a religion- unless you count atheism as one) so here's a shorter, yet, more strange example that gives me and pink_Lem0nade Vietnam flashbacks;

Selozar x Albert-What's wrong with being gay?

Probably the worst fanfic I've read in my entire life. No joke. Actually there was worse- but I'm not even going to talk about it...

Anyways, I can't explain what happens in that fanfic, because SO MANY THINGS HAPPEN AT ONCE AND IT CONFUSED THE FUCK OUT OF ME-

Like, they threw in Albert being transgender just so he could have period cramps, previous to this he got bullied for being gay, then decided to take A MONTH BREAK. AND CAME OUT AS TRANS. AND PEOPLE ARE JUST- OKAY WITH IT? I'm sorry, but that's NOT how the world works- AT ALL.

Like, then a chapter later Selozar gets the hinaki disease, and fuck I don't remember the ending. All I know is that I literally questioned my life for a solid 3 days after it.

DON'T. THROW IN. THINGS LIKE THAT.

EVER.

It makes the fanfic bad, one thing that I hate is when they throw... nekos... in...

Sometimes it can be handled well! Such as PurpleFluffball's neko story, it made some sense. I don't like her writing style anymore, so that's probably why I don't really have an opinion on it.

Going back to the planning your story ahead of time, going with the flow in writing usually ends in disaster. Remember my two awful high school books? Yeah, I had no plan for them, and they both got discontinued relatively fast. Think of a basic story line, and add detail to it. Just like drawing from a base you created- You make the basic outline, then you slowly begin to add the details and POOF THERE'S YOUR UNIVERSE! I'm open to giving you a basic outline of a story, but all though yes, right now it is free, but later in the future I'll have to charge something, or not give ideas at all.

If you go with the flow on writing, random shit happens because you have no idea what else to do, it turns into a cluster fuck, and the book isn't enjoyable anymore.