I believe it would be interesting and fun to have guild vs guild PvP mode.
team size may be 12vs12 or 16vs16. a guild can have more than one team represented. each game has 3 rounds. home, away and neutral. 2/3 wins to win the game (play home and away first and neutral last).
@ home, a team gets to choose its preferred map (new maps specifically for GvG). there are also cosmetic options for teams to customize their home map. the 3rd round to be played in randomly selected neutral map. Ranking system for this mode, we can consider other game models such as LoL elo or perhaps wow arena-like system.
Just an idea, ofc. Something along this line would be a great addition imo. Guild pride etc.