Could have quests or rewards or weeklys as well to encourage grouping for these further along with relevant gear or rare item drops. Could have rare sets to collect by downing multiple different bosses across planets.
Some could have unique challenges to master. Speed kills, immune to XX, need to kite to another location a distance away to kill, boss only spawns after killing another world boss or side quest, do with only xx players .. sure there is more possibilities.
Custom sets from each boss would be ideal and changing up the fights would be really rad. Like putting one boss on a cliff, then giving them knock back attacks. Changing things up in a simple why, but adding a challenge that would really incorporate some world pvp.