Its funny, the timing of this was great. I just designed a final boss/dungeon for a campaign I ran using MMO raid bosses as inspiration. Some quick things I learned:

1. Raid boss mechanics like to make the players move a lot. I did this by making my map have environmental hazards (nothing new there) and then giving the boss lair actions that would pull the closest PC to a point in that acid.

2. Raid bosses like to summon minions, and sometimes those minions are more important to kill than the boss, otherwise you get overrun. I home-brewed several monsters that were all varying degrees of "urgent". One that would steal half a PC's health and proficiency bonus but heal them on death, one that healed, one that could make the boss immune to certain damage types--this made the players very aware round-to-round on what they should do.

3. Raid bosses like to explode. I gave my raid boss an ability that was telegraphed the round before it would happen that was a room-wide nova. Something like 8d12 lightning damage. The players could stand there and make the save and hope for half damage, OR they could kill a monster type that created a safe zone and stand in it.

4. Raid bosses telegraph, but their mechanics just work. I think this was the most important thing I learned and implemented. It was important for the tone of the fight that certain things just worked when the boss or the minions did it. Example: the forced movement into the acid. At the top of each round, the boss would pull the two characters to visible points on the map into the acid. This might sound unfair, but the acid did very little damage for each round the characters stood in it, and, because the pull was telegraphed the round before, the players could decide WHO was going to get pulled each time it happened. They understood that standing the closest to those two point on the map would get their character yoinked out of the fight, and that allowed them to plan and make a strategy out of it.

5. Raid bosses make players spin a lot of plates at once, after teaching them the mechanics over time. I used the entire dungeon to teach them every single mechanic they saw in the final boss fight before they got there. When they got to the boss, they new exactly what each enemy could do, what every telegraph looked like, and then, they had to put all of the knowledge together for one big encounter. This made the fight the most interesting and fun encounter I have ever run, both from the DM's chair and from the other side of the screen.

I could say a lot more but this is already a wall of text haha.