[Stone Mill]

Consider this:

Start with two "tribes"; one game dedicated to each. Each tribe must eliminate it's weakest member.

At some point, the tribes must merge, and then there will be a free-for-all. However, as with the show- alliances will preserve within the tribes. You will see that cross-tribal alliances will be rare. After playing 10 or so rounds of "you get my back, I'll get yours," you will tend to trust that person when the two tribes merge into one game, rather than some fresh meat. Besides, you will be suspicious and paranoid (and rightly so) of the alliances formed by the other tribe.

Another thing to add as a feature to this game is "IMMUNITY." If you are the player who successfully eliminates the weak race, you get to sit out the next round. That will provide incentive for aggressors, rather than having everyone adopt long drawn-out defensive postures. That should help hurry along playing time.