How about adding bonuses for things like most troop donations with in a season and add a special banner next to the name or shield, until some else wins it the next season. Or most trophies won, in multiplayer, with in a season. Could also do bonuses for most DE, pink and gold or most th destroyed. Thing like that. Basically the same type of incentives already in the game, but make it a seasonal thing in addition to the one time achievements.
