Rankings move too drastically after each game, particularly after coming in 6th place. I've played 700 games.....coming in 6th shouldn't drop me hundreds of ranking spaces! The ranking should use a much longer view of average finishes. As the rankings work right now, its next to impossible to remain ranked as a Grandmaster, because even a 2nd place finish in an average game (2 novice, 3 beginner-experts) drops your rank a few places.
Which gets me onto the next point- you should have the option as game initiator to hide your rank level. Anyone who knows the game of Risk, knows that if someone wants to go kamikaze against you, you can't overcome that. And that often happens to me as a Grandmaster.