Matchmaking should not be across divisions ever. Keep it simple CG. Lock a division and do matchmaking off of GP within your Division and League. Create fake accounts that will fill in when a Division/League combination isn't divisible by 8 (don't cross Division/League buckets to fill in the gaps). The matchups will become "more fair" as the championship progresses and people fight others with similar win/loss records (as determined by league progression). This should not have been complicated.
This recommendation coming from someone who has the highest GP (i.e. "bloated roster") and is still expecting to win the GA.
