Hmm. It's difficult. I'm not sure if a rotation would be good, especially if it was weekly. If there's a game mode that I just don't like and that's the only one playing in Ranked, I'm not going to want to play all week. A daily rotation could work out a lot better. The variety remains but it's changed up more frequently. That, or there could just be a vote before each game starts. Most shooters tend to go down that route anyways. Only problem is that it sucks when you want to play a particular game mode but nobody else votes for it multiple times and you never seem to play it. :(
If none of those, maybe it should just be randomized every time you play. Of course, then you may have people quitting the match if that happens. Though I suppose that could happen even in the voting scenario.