umm ... perhaps its just me thinking but define a boss before deciding a matter of fighting him, i would recommend a story for say ... 8 sinister beings /demons and to fight one of them you gather relics in their "area" in which they are affecting the world.
Say Tulimshar has a Fire sinister demon (the toughest boss in Tulimshar realm) to reach him you start out to ... say 5 dungeons to gather these relics (5 of em)
After gathering the relics you go to his "altar" a dungeon perhaps in which you must travel to the deepest part (a very hard thing to do) and open a gate to his plane and fight him ...
this way anyone can go and fight him ... yet doing so will take time and effort you cant camp him sicne you must get the relics again to open the gate
To even make it harder to camp a boss you can add a system like the portal to their realm is only open one week named after the color of that weeks moon ... and one portal is open every week but they rotate depending on the color of the moon, for that a NPC could be implied a Mystic or something that says "This week the moon is bathing in fire" and so its the Fire demons week
Fire demon - Blazing Moon (lasts one week)
Ice Demon - Freezing Moon (same as above)
etc etc
for "mini-bosses" i would asume there is a story behind them, like Firedog of (name of Fire demon) or some such and those monsters could have different rooms to spawn in (decided spawn places which is randomly decided) like every dungeon has 4 corners in one of those corners they spawn ... and if you also add a spawn rate of 1 hour the adventuring groups must travel the dungeon from corner to corner to first find adn then try to defeat.
It is also a nice thing to add that when a boss of greater importance has been defeated a realm notification is spread in teh cities, like a trophy from the adventurers (as World of Warcraft has with the dragons, Onyxia's and Ragnaros heads are put on stakes to show the people that they have been slain) perhaps the Horns of a demons....
Well either way this is alot based on Story line Bosses and that they are in some way related to Astral planar (which can explain why only one group is at the boss, since they ventured there thro the portal together)
For other bossess perhaps having them flee and leave treasure chests is one way to keep them alive without respawn or summoned back to their "masters of Immortality" to then later reappear in another region...
This way you can have them respawn with their "army"/guards in any abandon camp that is spread around the realms from teh battles between the Gods and mages...
I strongly recommend that you first decide in what way TMW is to lean towards, a fully Community (more RP) mmorpg cannot have the same competion like a PvP oriented game ... since then that would be a game of perfection and remakes and .. i cant imagine how many hours of freakish slavery that is behind that game either ^^
*sighs deeply* and yet again i have proven my bastardness in typing way to long comments -_-

well if you have the time and patience to read through this post without fallign asleep i think you might have seen more options in creating bossess.
*short said, some specific Awesome bosses should be group only experience (no one else can interfere) yet alot of other bosses (mini bosses, half boss (like a captain or somethin)) could be a more competetive between gamers for finding and slaying or so i believe