This is an idea that stems from a few other RPGs out there: Reputation.
Basically, killing certain monsters or bosses grants you units of "reputation" for a particular faction, or takes points away from others. For example, killing Rel-Thon will grant reputation for the "Nightmist City" faction, while killing the enchanted sword (your prize after you kill enough Bandits) will grant you reputation for the "Empire of Harabec" and killing the Time Knight will grant you reputation for "Windia", while on multi-alt, completing the Inquisitor quests will grant you reputation for the "Inquisitors", but also reduce your reputation with the cities that their targets are associated with.
Having a good reputation will allow you to enter certain shops and areas, as well as being extra points to collect that are visible in your look-over or something. At the same time, having a terrible reputation with a certain faction may allow you into the base of their mortal enemies or something.
But here's the catch... when you kill a monster or boss (mostly bosses) that grants reputation, the entire party get the reputation reward, not just the crit that landed the kill-shot, so there is incentive to team up even if you don't stand much of a chance of winning the main item prize, or already have it.
It may be a bit too in-depth for Nightmist, especially considering the realm has been designed without such reputation for a while, but it may add an extra layer of depth to the game - what do you think?
Other minor examples...
- Killing Lion Cubs will drastically reduce your reputation with naturalist groups (e.g. the Coven) because of breaking the rules of the hunter (pregnant females and cubs and their mothers are off-limits). Raising this reputation again takes effort, such as from killing Skeletons (which give much less reputation).
- Clearing out undead from desecrated locations such as the desecrated church will increase your standing with spiritual groups, while killing the innocent and defenceless (Mourners, Wounded Druid) will reduce it, as well as make city factions wary of you (after all, you are murdering the innocent).
- Having a good reputation with Resthaven (for example) will allow access to Megron's Amulet Shop, while having a poor reputation with the city stops you from using their tavern.
- Some factions are almost impossible to gain reputation with (e.g. "House of Ku'Nal"), so infamy with such factions just shows the playerbase how much you hate them!
Side-effects hopefully include:
- Encouraging team-work so multiple players can share high reputation gains from bosses.
- Encouraging more diverse training by hunting for monsters that give reputation to particular factions.
- Encouraging parties to fight the monsters along the way to a boss, in case they give small reputation gains themselves.