I honestly don't have anything outside of a vague idea of what I want for the story. I personally feel really restricted when I plan things out too much in advance, because that's just how my brain works, so I mostly come up with things as I go, writing notes here and there when I get ideas. I don't even have one of the major characters in one of my comics fully designed up since they're not set to appear awhile yet, but I know their purpose they'll be serving in the story and why they'll be hanging around.
But yeah I don't think anyone should really sweat too many of the details for things that are far off when they're just getting started. I feel it can potentially turn into a "reason" to not just go ahead and just start working on a comic if people aren't careful about it and try to figure out every little detail of every little thing ahead of time.