The "Alien, But Still Makes Sense To A Human Audience" parts are usually where I get tripped up with my fanfic worldbuilding. It's a PAIN to try to establish how they're different from humans but still keep them understandable. (Which is why, at the moment, the extent of my worldbuilding for one big AU is "a handful of notes on robot superpowers" and "a document of Cybertronian slang that amuses me".)