- Free roam co-op of course. Basically people can help you exactly where you are in the game.
- You can make the game open to friends, everyone, or no one using the settings menu.
- You can invite and remove people as you please.
- Players must be given permission to interact with items and townspeople. Someone who doesn't have permission can only fight enemies. This ensures that you won't have to worry about stolen items or slaughtered townspeople.
- Friendly fire can be turned off and/or duels can be initiated.
- You can trade items and equipment with people.
- When you die, you are given a short amount of time to be revived. If not revived, you teleport to your partner. If you both die, you will both start at your nearest checkpoint.
These are just a few ideas thrown off the top of my head.
I personally think it's a great idea. I also feel like it's a long shot away before we see anything like that happening.