It is really hard to say what you might need, because everyone's perferences and/or laundry routine is different.
If you do laundry often, you can get by with less.
You are definitely going to need some sort of covers. You mentioned that you like natural fibers. Do you knit or know someone who does? You might really like knitted wool longies, shorties, and soakers. Maybe your coop has some?
For a baby registry, I would keep it simple and focus mainly on the prefolds.
Since you don't like the polyester in most AIO's, but you really want some AIO's, you might check out some of the WAHM's diapers on hyenacart or etsy that make AIO's with mostly natural fiber inserts and linings.
~ help meet to my Lionheart, mama to 6, due in Feb. with #7