I've had this idea for a while now and it would be really useful to me.
By the idea 'Plot ranks' I don't literally mean, that if your on a certain plot that your an Admin on you can ban people from the game. By this is mean, Say your an Admin on a plot- Which the owner will have had to set you as an admin, You can Add people to the plot and Deny people from it. You will not be able to merge the plot, Clear the plot (You will be able to by hand but you can do that when added) or change the plot Message/biome ect.
This will be useful for Rp owners. If the owner of the plot goes AFK and somebody decides to grief the plot (Yes I know it can be fixed my staff but this will make the job easier and it's a quicker solution- Staff may not be online) an Admin of the rp / Someone who helped to build it (If there is one) would be able to remove/deny them from the plot if they were an Admin/Moderator of the plot and stop the grief before it gets worse. In this scenario it would only be useful if they caught the griefer but it may be helpful in many other ways!
Such as: I share a plot with my sibling, She doesn't come online a lot and I wanted to Host the rp we built together, But it was on her plot. She had left the RP plot open so that I could host it, But it had been griefed (And has been MANY times - Whilst we weren't even online, And when we have been online). If the plot wasn't open , it wouldn't have been griefed, As till this day we have never hosted the rp and it has been griefed the total of 7 times, I know it isn't much but considering we haven't hosted yet, It's a bit strange.
Sorry for the rambling, I hope you like my suggestion. Give me your ideas / Suggest why it could be used in more ways!