[breadfan]

Honestly I think that enforcing some kind of rotation or mix solves it, so I don't know where you're at with that, if you've discarded it altogether or are just thinking it over... my suggestion is (and this might be too convoluted/too much hassle, if so I apologise)

- Leagues assigned numbers, Bronze=1, Silver=2, Gold=3, Plat=4
- All players "check in" before the match (declare their eligibility) - everyone on the team MUST do this if they're online
- Both teams have their league scores averaged from the checked in players, the average of the lowest team determines how many of each rank you can have
- The lower team declares their team first, but they must not have more than 3 players in their lineup who exceed the lowest team's average
- The team with the higher average then declares their team, but they must not have more than 3 players in their lineup who exceed the lowest team's team average

Again you get in that ugly situation where you need to make a concession if a team has no choice but to field their 5 plat players because no one else is on, but the strength of this system is you could then award a handicap rating based on those scores. One suggestion that comes to mind is if a team has to exceed their 3 player over the average limit, they don't get the bonus point from the ace match (although it's not a throwaway game, because they can still prevent the other team from taking their 2 points!)

In practice:

ToR vs Zerocraft (100% of players online)
ToR - 8 gold, 4 silver, 3 bronze, Avg Score: 2.33
ZeroCraft - 1 Plat, 4 gold, 4 Silver, 3 bronze, Avg Score: 2.25

ToR is the favoured team, teams are not allowed to have more than 3 plat or gold players, ZeroCraft picks their lineup first, ToR picks second. Any number of bronze/silver are allowed.

This ensures balance and should facilitate rotation. On the downside, it is a lot more work to administrate, and suffers some of the same drawbacks as before.