Well the best fanfiction works are those that take the world created by another and add their own things to it that don't break the world, but still add interesting things to it that are your own creation.
You can take any world and create a story around it that doesn't completely copy the original. You need reasons for your additions and changes. Logic is something that needs to be nailed down heavily, so you can answer when someone asks about it.
For Warrior Cats, cats won't start randomly talking to humans. Cats won't be using calculators to do statistical equations. Fire burns flesh, and does not spread happiness. Ice is cold. Trees don't have legs. There are no Feline Space Pilots in the clans. (or at least none of these without a rational line of reasoning behind why this is so in your fanfiction work)
It is the same stuff you have to pay attention to when making your own world. If your world lacks any kind of flying life, then your reader is going to being confused and possibly pissed off when you mention a bird flying overhead with no mention of how the bird is flying overhead when this world lacks flying creatures. .
Of course, those rules really only help with readers with a decent sense of maturity. Readers that refuse change or that aren't that well-versed in literature might end up calling foul for changes in the original author's world.
Some of my favorite fan fictions ever were one that altered major characters or took a group into an extreme situation that they weren't used to. Others were ones that created their own things, but still respected the laws of the world.
