Prison Suggestions:
At /warp shop there are 6 available spaces which can be filled with useful items. The items I believe will be most beneficiary include the items below.
Brewing stands
Quarts Blocks
Slime Balls/Blocks
Red Mushrooms/Brown Mushrooms
Mycellium
Vines/Certain Types of Saplings
Items I listed give no unfair advantage and helps players when building their plots or fighting on Prison.

Another Suggestion: Mines which contain: Woods of Various Types, as well as leaves and Snow Blocks.
Another Suggestion: Craftable Haste potions which can be used to Mine/PvP.
Another Suggestion: Plot merging, such as owning a 4x4 plot area, and having the ability to destroy the road in between.
Another Suggestion: Different types of monster spawners at /warp spawners on Prison such as cow and sheep as well as already added pig.
Another Suggestion: If a player hasn't been on the server in months and hasn't logged on and is not banned, you may be able to claim their plot however, the player's chests will be saved into the server's files and when the player comes back (hopefully) s/he will be able to type /reclaim which will come up with multiple pages of his/her lost items which he/she lost while inactive. This would mean that plots near you will be free, and you can expand your plot.
Another Suggestion: Fortune increases your amount of blocks mined at any mine.
Another Suggestion: Daily/Weekly drop parties containing an efficiency 11 pickaxe.
Another Suggestion: Chest Shops, the ability to create a shop using a sign and letting others buy/sell items to and from your plot.
Another Suggestion: /p h (Username) allowing you to visit other plots if you don't know the exact coordinates or your friend is not online.
Another Suggestion: Auto Sell, /AutoSell Yes/No enabling a feature where any block mined that can be sold will be transferred into tokens automatically saving time for those who are on a mining spree.