A quick suggestion that I'd like to see at some point is a planning tool.
Having played quite a bit of Prison Architect, I really do find this tool invaluable, it basically allows you to put down a basic plan without spending so you can forward-plan your areas and ensure they all fit neatly before even beginning building.
One thing I always wanted in Prison Architect was a real to-do list where I could write notes to myself and check them off as I complete them. Sometimes outlining walls isn't enough, and I'll forget what I was doing several hours ago, or days ago.
Notepad sounds better than planner tool. Much better actually. And it's MUCH quicker to do.
Planner sounds nice in theory, but in actual game it's next to being useless (yes - I do play PA too) while most likely it's development will take quite a bit of time - time that can be spend on improving the game instead.