How about a system where an item needs to be traded to a location to spawn a boss, which then belongs to the party (or alliance, if possible) and scales in difficulty depending on the number of party members? The item could be obtained from the treasure chests mentioned which require exploration, and can also be unique to each party/player so groups aren't fighting over chests. Tie it to a system of map hunting, quests, and/or leves, and it could be a lot of fun, potentially.

I think the main problem is that it is not "open world, anyone can jump in" content, but between hunts and fates, that kind of content seems to have a lot of problems anyway. Why not try something like this instead? You'd still share the open world aspect with others, but you could do your own thing or group up with randoms as you please.