In Halo Wars they had a difficulty where it was adaptive. Every time you beat it it got harder. Personally every mission should have an adaptive difficutly. Where it's base difficutly would be 20, then with every time you beat it it got a bit harder. If you won it would increase the level of the mission and if you didn't beat it or gave up it will decrease your level thus forcing people to try and what not. As for the rewards and which missions will have this feature it will be up to bungie. Let's say a person has beat a few levels and got it to adaptive 30 and they have a friend on adaptive 28, they can join together and only do adaptive 27 and below because you won't be able to have a fire team in the players current adaptive level they are at. This will force players to train, get stronger, try new strategies, learn how important teamwork really is, and other things.