If I wanted to do a run for an item that is not on the list, would I just upload the run to my YouTube and submit it like any other run, or would I need to contact one of the staff?

You can run getting that item, it just won't go anywhere on the leaderboards. It would only be for your enjoyment. But we'll happily take suggestions for categories that don't exist yet, just no guarantee they'll be accepted.

I think most items would be pretty fun to run. Brick block for example would be good since it would require more effort to get. Also, i rarely spawn in or near deserts so a cactus run would probably be a bit of a pain, but rewarding