It would be incredible to be able to create custom groups in addition to Age, Gender, and Location.  This would be extremely useful for A/B testing various features and very naturally being able to compare all metrics for two groups in the A/B test.  For example, for each user I could provide a registration event for "TEST1" with value "TEST1_A" and another user with "TEST1" with value "TEST1_B" and then I could filter results everywhere I could filter for Age, Gender, or Location with my arbitrary group.