Open world contents can be fun & fair at the same time. Take GW2 for example, usually there would be a chain of several dynamic events (FATE) leading to the main boss one, so people have time to gather & be ready when it happens, while impossible to rush early. Some bosses require people to do different events at different places on the map in order to be spawned, as part of their mechanics. Not to mention events being scaled with number of people participating, but I guess that's impossible for FFXIV as it was suggested since 2.0

If the dev still find it hard to do, the simplest solution in my opinion would be giving hunt bosses a 5-minute Invincibility buff, so no one can kill them, and would be killed instead if they start pulling early.