Spread out 5 bosses around all of the different worlds.Make them only there in hard mode. Make them insanely hard, like lvl 40 or something. Surround them with difficult mobs. IE make them hard to beat for all but the most organized/skilled teams.(Similar to what you did with Rotscale but much harder)
When you kill each of them you automatically get a special "crafting" item that can not be traded. When you've collected all five you turn them in to some npc(maybe through a quest, maybe not) and recieve a unique armor set. Call it Conquerer's armor or something of the like. Make it like 15k armor priced.
Obsidian armor shows that you got wealth. This could show that you've been to the different continents and beaten the best they have to offer.(You could also just make this all part of a quest and do it that way. The bosses only appear if you have the quest and you have to illiminate all five for some npc to be willing to craft you the armor)
PS: even if this idea is no good you should still make some multi-continent armor set that is somehow very difficult to get. rather than just costing money and farming time. something that shows it took some skill/teamwork to get.
L

. Everyone keep posting their opinions and support and maybe this will get picked up.