I imagine those with the richest past would be rewarded with a legend thread in which they could continue lingering.

For the rest, it could be a fun journey back to official - Reinvigorate the clan with new enthusiasm and new blood.
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Though, I would suggest instead of splitting in-game into 3 distinct categories (XP, wars, in-game activity), you combine in-game activity with XP or combine in-game activity with wars. Wars make up the vast majority of XP gained, so essentially wars are counted twice over. Not all clans enjoy warring, so I feel it's a bit harsh to punish them if they don't war - Instead, just transfer those points over to general in-game activity. Give a think at least ey.