Currently guild are rank in according to their guild level only.
I would suggest more guild ranking available and a small rewards to the guild who won the individual category
example of it will be
1. Gauntlet - Guild high score. (but must implement this first;http://community.gtarcade.com/post/87479/)
2. Diamond Guild - Can be the total amount of diamond a guild spend in that month
3. Guild power - Total damage a guild do to world boss in that month.;
4. participation - Total number of player login in that month.
Hope you consider the suggestion. thank you