The dev post of rate these bosses on difficulty made me think, about what would be the best we can think of as a boss for this game.
The following comes to mind to me. The boss needs to be able to involve everyone in the fight. If we traditionally go to a fight with a tank, healer and dps, then the following comes to mind to me:
Tanks
The boss needs a tank buster for the tank to watch out for. This tankbuster will only start instakilling on higher shards to make the mechanic only nessecary on lvl 7+ as example. This is important to make it scale up like that.
Further more to distingiush the boss it needs a seperate mechanic for the tank to deal with. This can be dodge away from the line of sight of the boss something else like that.
To tackle the lack of warriors we need to address one of the big advantages of pally, stunlocking. The perfect boss does not allow for stunlocking this much as it makes the warrior obsolete. Prevent getting any major advantage from stunlocking as pally.
What about adds? Same deal, warrior is in the major disadvantage if we are talking adds spawning as the pally stores aggro bursts and AoE aggro groundslams on multiple adds. Would avoid adding adds to a new boss to the current stage of the tanks if the adds are meant for the tanks to take.
Healers
To tackle the most neglected necesity of the 3, the healer, it needs its own separete challenge. A healers task is what? Healing! Does it have to be to make the job easier for the tank? No! It should be a mechanic only a healer can handle.
An example: there is a bomb selected on a random player. Its a very clear indication who got the bomb (not like the death icon from old game tradu boss, that one was too hard to see). Like a yellow glow on the player? They will start taking damage after a while and the healer needs to focus their heals on that person. Sadly, everyone standing close to that person will be glowing red to warn that standing close is a bad plan. They will die if they stay there. Because just bounce healing everyone blindly is not going to be the solution with people so far apart. it will kill the other people standing close to the target player. This way the healer job is something else then spam bounce heals on cooldown. It needs to be tuned that a bards crescendos can save the player from the damage but just their passives cant. And of course only on higher shards is that how much it damages 
Dps
For the dps its more a question of will the mechanics make the fight more fun or just frustrating? And is it fair between the dps.
Always question when the dps are being forced to be moved somewhere if one of the 4 dps gets a major downside from this movement in their play? Will they feel like they are contributing less because of this? Extreme movement should always be paired with a down phase where you cant dps. Otherwise shamans will be left in the dust. And the distance between the boss and players should allow 30 meters as for rangers for this to be fair.
Sadly those two classes are in major conflict in needed distance. A possible mechanic is having the boss start teleporting around in a predictable area stopping long enough for shaman orbs to reach on time once while staying in an area that keeps the boss 30 meters away for rangers.
Then it does not always have to be about dps itselfm There can be a mechanic that a dps needs to do something else, as long as they dont feel like they are secrificing dos for this mechanic, then it can work out. It can also make people feel more included in a fight beyond their dps. Which is very important for a healthy community.
Some other notes
- As long as the above conditions are kept you can go wild in creativity of a fight. Remember, in the end the goal is to engage the player. Which will be achieved if you make it fun for them and make them feel like they matter with what they do.
- Mechanics like AoEs should have clear death area. Otherwise frustration of unfairness happens and the mechanic should have just been avoided. Being on the edge of an AoE circle not showing you the “going to die arrow” while still dying is a current big flaw in Orbus.
- The line swooping AoEs in current boss fights are flawed as the client and server agreeing with the moment of the jump differs widely. Giving an unintuative moment to jump over the AoE. Also this jump moment changes based on internet ping which makes it unfair. Then we have the problem of when the ground is not flat you dont see the AoEs well (cough crab boss)
So here is my two cents of Orbus boss fight design 
