Making the respawn timers more random would kinda help but you'd still run in the issue that if a mob respawns during euro primetime then the US players probably don't get the kill, and if there's a bad streak on some mob that happen to drop some specific really good loot it kinda sucks you never get a chance at contesting it ever. Shared loot tables between contested raid mobs maybe?
Other way to do it maybe would be to have a boss that's almost always up at any given time, but the locations would vary a lot so you'd need to look for it and organize your raid asap to contest it, but even if you miss it, say 30mins later another boss spawns somewhere in the world. Would require absolutely no static spawns for these though so you don't have people logging on mules to check the various locations but instead actually have to look around. Mobs would need to be untrackable and the world would need to be big enough for that to happen. Or I guess you say fuck it and that people shouldn't be expected to get whatever they want, which is also an option.