Players as mercenaries?
Interesting, i am not 100% with you but i got inspirited by the term “Mercenaries”.
What i suggest is to make a mercenaries party event, which any player can go to the unfinished heart quest giver to launch mercenary event, other players can access the mercenary job list anytime to find jobs, they can click the event and add themselves into the request player party as a mercanary. No matter the mercanaries finished the heart quest or not, the reward upon success will be a few coin (by %) and a new token “Mercenary trophy”. Which is able to exchange some mercenary chest (or some skin) from npc and get random loots like the black lion chest.
This way whoever got nothing to do, can browse this list and earn some money by finishing the “done” heart quest again by helping low lv players, and this way can encourage high lv players to going back to almost deserted low lv maps.