That's a pretty cool idea
/ban/ - a board that only banned people can post on. When you get banned it says "you have been banned, blahblablah you can still post on /ban/ which is an exclusive board for banned people"
Although I don't know the real use of it…like what would be the point of posting on it if nobody else is banned? Barely any people get banned…w/e
Or maybe a graveyard like what fringechan has, all the deleted posts (that aren't nsfw) are put on the graveyard and if you want to see an old deleted post then go to /grave/