Create a clan battle system, where clans can enter private lobbies to battle each other for ranking positions on a global leaderboard. Make it so that everything from kills, vehicles destroyed, highest sniper kill range etc all contribute to the position you currently hold on the leaderboard. Final thought is that you can allow clans to challenge one another for the position they hold in the rankings, but the clan being challenged can only decline 5 challenges in row (doesn’t have to be from the same clan) before they have to accept the following challenge.
You can only challenge clans directly above or below you and you gain points for defeating the above clan or defeating the below clan, clans can also gain points by just playing outside of clan battles as well. This would all occur over multiple seasons where prizes (cosmetics or whatever) are given at the end to clans according to their accumulated points.