I think also achieving such a high enough bounty should provide titles or single claimable rewards like say you finally hit A-Rank or you get S-Rank for the first time you get a reward or the person with the Highest bounty gets a title that people compete for like "Worlds Deadliest"
In terms of Other mission types that involve Event coupons, probably new areas with harder bosses that respawn so slowly they in themselves are called events. Or every so often in different areas a band of 5 tough bandits appear somewhere in the world and they have a normal mob drop chance to drop a event coupon.
Maybe make new quests involving items that arent currently beneficial like Bandit blood or the Snow Bear Paw or the lost snake or a quest involving all the different incest parts (Scarabs, bees, Scorpion, Ants) that creates a Insect Chimera boss
But i agree that the game needs something to bring more people out from their idle spots. I dont like seeing all this open space and every time i pass by its a ghost town.