I think if we are going to keep the "Prison" of Prison Architect, I would really like the next game to be about a penal colony.
Make it a Sci-fi/punk/dark fantasy setting. You are running it on the behalf some morally-dubious-if-not-outright-evil empire/megacorp that's using the prisoners as labor to build the colony up and keeping it running. Challenge isn't just keeping them housed, fed and in line, but also getting them to harvest, build, and craft almost everything you need to grow the colony, balancing the freedom they need to preform those jobs and the security of limiting their access to contraband. Bonus points if it keeps track of prisoners who escape that might form a band in the wilderness that raids the colony now and again.
In general though, IMO Prison Architect, and what an Architect IP might stand for, is a game where;
-You are in control of a single installation/complex/building.
-You are trying to serve the needs of a population you cannot directly control, with a smaller group that you do have limited control of.
-Said population can and will do things that will undermine your objectives, requiring you to enforce some amount of control over them.
-The result of what you've build should be something you can have fun with as a member of the population you are trying to serve/control.
-...And the tutorial/campaign questions your moral responsibility in all of it.