The template is Ragnaros in Molton Core. However, other heroes like Deathwing could be used. I also imagine legendary figures that haven’t made it to HotS, like the Old Gods from Warcraft, would fit nicely here.
The map would be unique for this mode. Most likely circular in design, possibly with camps around the periphery. Some bosses could be more or less stationary like Ragnaros, but others like Deathwing could be free to roam the area. Minions may still spawn regularly as a way of generating XP, but perhaps they should be tougher than the normal game modes. Alternatively you start at level 20.
I’m not sure what the correct number of players would be, but I think it could be really fun for it to still be PvP with the Raid Boss controlled by a player.
The game needs some kind of tension for the normal players. Possibilities include limited respawns, a time limit, the ability for the raid boss to destroy the player hearth. Something along those lines to prevent it from being an infinite respawn marathon to eventually kill the Raid Boss.
What do you think? Is it a mode you would be interested in? Any suggestions for improvements?
Can you tell I miss brawls other than ARAM?