Some other MMOs have the concept of map contribution, which means that you have to contribute so much to an event/meta event before you start earning the better rewards from it. And the best rewards are only handed out to those who reach very high levels of contribution/participation in the event. I think that a similar system for elite boss rewards could take away a large part of the toxicity and camping that surrounds elite boss hunting.
In short, you can only obtain contribution if you are in the map's level range. And for mobs below level 200, if you are more than 5 levels above the mob you get reduced contribution. Right now I envision contribution as being a mix of killing mobs in the map to help spawn the elite boss, and also damage dealt to the elite boss. This way, players who elite hunt can still get a tiny bit of map contribution, but the players who actually put in the work to spawn the elite will get the better rewards.
If you have 0 map contribution when the elite boss is killed you are ineligible to receive one of the rare reward chests. If you only have low contribution, then you can receive a new version of the chest which only a small subset of the prizes. Probably something like no CSS and no epic pot scrolls IDK. If you have high contribution you earn the regular, purple rare reward chests. And the more you have, the more chests you earn.
To make it work, it would have to be time gated. What I mean is that you would have to be in the map, actively killing mobs, for several minutes before you start earning contribution. This combats those KSers who wait until a map is dark and then take all the kills to spawn it. They shouldn't be getting anywhere near the same contribution as the players who actually put in the work to spawn the boss. Also if you wait too long without gaining any contribution (10 minutes) then it starts to decay. This keeps people from going in, killing several hundred/thousand of mobs to get high contribution and then just camping in the map afk on a platform safe from the elite boss and gaining lots of rewards for doing nothing.
I know that some will be against this because they hunt elites to gain maplestory 2 mesos. But that mindset is so toxic to the game. People should earn rewards through actually playing Maplestory, not channel surfing and KSing someone's map when its about to spawn an elite. That's just so incredibly toxic, and this would go a long way towards getting rid of that mindset. If implemented properly they could even make the reward chests better since people would actually be earning them.