For friends that need to measure manliness a dueling area would be sweet. Maybe use the iron banner area for challenging ppl to duels as an interact option. Just make lord saladin appear or vanish as nessicary. Just a thought.
I actually thought a lot about that myself, and came to the conclusion, if done right it could be really awesome. If not though I feel like the feature would be hated on and abandoned. Personally for me, Halo was a lot more fun with four or more people, it was just too many deaths for a fun one on one experience