Since has a lot of horde mode missions, why don't you make a survival mode? The prison looks like a perfect opportunity to make one. Here's a suggestion:
Your team of 3/6 guardians (It would be cool to have 6) will enter the prison normally, Variks will say: [i]"Welcome to prison of elders. Choose your enemy, take your stand."[/i]
All 4 gates will open. Your fireteam can choose any enemy to fight. When all guardians enter the arena for that enemy, round 1 begins immediately.
Enemies will start at lvl 20. They will constantly get stronger as their lvl increases each round. Enemies will gradually earn yellow bars. More and more will spawn each round.
Guardians can't be revived. Self res allowed. Every 3 rounds, your entire fireteam is revived, but a modifier (any) will be given for the next 3.
Here's the nice one. Every 10 rounds, the gambling round will be offered. They're 2 things you can do during this round:
A. Take the gambling round. All modifiers will turn on, with the exception of small arms or specialist, only one of those weapon modifiers will turn on. All enemies are yellow bar, and all enemies that have a shield get one. This is the hardest round, but will also grant you an exotic weapon/armor, if completed. Hardest challenge, greatest rewards.
B. Skip the round and continue normally.
Variks can say dialogue, but I didn't really think of any. Destiny would be better if it had some type of survival.
Survival will be available every day of the week. It can be entered with any amount of fireteam members. Note that the same amount of enemies (A LOT) always spawn, regardless of how many guardians you have in your team.
So. Bad? Good? Absolutely Terrible?