I think it'd be cool if there was an end game boss in the game that was designed to challenge parties who have the best available gear. And this boss would drop class-specific legendary items with a 1% drop rate.
Possible Location: Maybe add a new wing to Vault and call the boss "Deep Queen". Or maybe put an island somewhere and make the boss some other unit type (like a giant water elemental).
Accessibility: I think it'd be nice if the boss was inaccessible until Deeplord, 15 Hydra, and Naga Queen were killed.
Boss fight: Here are some ideas of how the fight might be:
The boss would have enormous HP pool (at least three times what Deeplord has), because it should be only killable for parties with well geared dps
The boss would have enormous auto attack damage (at least double what Deeplord has), because the boss fight should promote using CC and damage reduction effects like Necro's cripple and it should also demand having a well geared tank.
The boss should put a damage over time effect on all party members which ramps up (1 stack, 2 stacks, 3 stacks, etc). This would create a soft enrage effect.
The boss should summon an add every 10 seconds, which need to be tanked up. When killed, each add should remove 2 stacks of the DoT from the party and restore 50 mana to all party members.
Whenever someone uses a HP pot, the boss should heal for that much mana as well.
Boss periodically calls out someone's name and places a debuff on that which causes them to explode and instantly kill all members too close to them.
Dropped Legendaries
I think class specific legendaries should be unnamed and then first person to post a legit reply of them getting the item should get item named for them in future patch. That'd be sweet...