[*]Needs. Every prisoner in the sandbox has a set of needs, required to keep them happy. I'm not sure if all of them should be implemented, but the main ones, such as hunger, bowels/bladder, sleep, hygiene etc etc. Each one should have some effect that happens when it's need is near/at maximum, such as losing heath when food is low; falling asleep when sleep is low; peeing/shitting yourself (max low hygiene) when bladder.bowels is low, and possibly getting illnesses when hygiene is max. Drugs and alcohol addiction is also something else that should be added, however i'm not sure if it should lose health or cause some other symptom... I'm sure it's possible to implement all needs, but at least keeping up the basics would make it a bit more challenging. A (relatively) happy prisoner is a hardworking prisoner. Those tunnels won't dig themselves!
[*]Rep points for every escaped prisoner. You should get a hefty chunk or rep points for every prisoner you break out. Maybe start it at 10/20 points a prisoner, with a multiplier for every one you break out in a certain period of time. This leads to the next suggestion...
[*]Serving your sentence. Every prisoner should be able to serve your sentence. You will get no rep points, and lose the prisoner from your group (released instead of escaped).
[*]Gang Allegiance/Activities. If you are playing with gangs on, you should have to join in any gang "plays" etc that the leaders set up. Having a gang allegiance would help your crew protect themselves from other gangs, or you can play independent of gangs and hold a neutral stance (ie get beaten up or paying bribes to all gangs.
[*]More in-depth contraband and in-game economy. There should be some form of in-game economy. I should be able to sell knives from the kitchen, and sell them to by exchange for wooden pickaxes from someone who works in thenworkshop, or maybe break into the armoury and sell off shotguns to the highest bidder. And if i don't feel like stealing contraband, i can always make it by working (mopping floors, working in workshop, cooking). Each prisoner has his own bank balance, although there could be a shared one for the gang, although it's not necessary (we have rep points). Some form of in-game economy should be implemented and provide some form of supply and demand, possibly making your time in prison profitable
(just thought, this could be tied into the rep points received for every escaped prisoner).I hope some or all of these suggestions are useful and inspiring.