There is a fundamental flaw in all ranking systems.
You can rank a team OR a player, however in online gaming you are not limited to playing for one team, so ranking the team is not going to work. If you base the player rating on the actions of the team, or based on the ranking of the other team, you open the door to multi account and deranking.
These both throw off the system, and if a large enough population practices this, the entire system breaks down.
Personally, I think the best way to pull it off is going ultra simple.
Win a game, go up a rank, lose a game, go down a rank. Screw this 50, go to 100. Tweak trueskill and let it do it's thing behind the scenes to try to get the best quality matches it can, and DON'T tell people how it works. Once the formula is revealed, someone will figure out how to exploit it, and then publish that information for the world to see. What you have then is a break down in the system like we have seen in all previous Halo titles. So here are the bullet points:
Rank is Global. If you are a 90, you are a 90 across ALL playlists. This means you can't purchase a 100 account in doubles and then not play on it.
Keep the H3 experience "rank" system for each playlist
Hide the trueskill formula
Simplify the ranking system
In closing, the "top" rank should be reserved for a handful of players, worldwide. I'm sorry if little Johnny thinks he should be a 100, he is only a 45 for a reason. Time to grow up and realize we can't all be winners and the losing team does not get a trophy for trying...
-KI