I had an idea to think of a mechanic intended for mythic difficulty only that would require good voice communication on the fly, something that boss mods or raid markers couldn't handle.
It may be against Blizzards design philosophy to create abilities that can't be detected by boss mods (not show up in combat log), but I think it would be a breath of fresh air to have something that required some real coordination without a boss mod holding your hand.
So here's my idea, with placeholder names and some notes in italics:
Doom Laser: 5 second charge up time, 12 second channel.
Targets someone at random and deals extreme damage to them, and anyone caught in the beam every 0.50 seconds for 12 seconds.
Mitigated by Forcefield ability. Taking a single un-shielded tick will 1 shot you, even through CDs/immunities.
The 5 second cast time is where your raid has to communicate how to deal with it
Forcefield: 5 second channel
As Doom Laser begins it's charge up cast, three random raid members gain the extra action button ability forcefield (good timing eh? someone can think of a more plausible gimmick I'm sure)
Forcefield will reduce Doom Laser damage on the target by 99% while active.
You have to chain the 3 forcefields (with room for 3 seconds overlap) in order to cover the 12 seconds of Doom Laser.
During the 5 second charge up cast for Doom Laser you have to identify the target, and have the 3 random forcefielders work out who is shielding 1st, 2nd, and 3rd.
notes: For the purpose of this ability, the raid members who receive the Forcefield ability would have to be undetectable to boss mods - I'm sure that's possible to hide something from the combat log. Not necessary, but to make it extra spicy, the target of the Doom Laser could also be made unknown to boss mods, with the target having to quickly announce that it's on them so people can move away and the forcefielders can target you.
To prevent a possible tactic of having 3 raid markers that the forcefielders quickly move to which designate who is going to forcefield in what order, you could have another boss mechanic which encourages spreading out in the room - so there wouldn't be time to reach the markers.
I expect there would be two ways of handling the voice comms; the forcefielders quickly announce themselves and the raid leader assigns 1,2,3 or the forcefielders work it out among themselves and deal with possible conflicts if two or more people call the same number.
That second way reminds me of how our raid dealt with assigning interrupts for empowered MCs on heroic Garrosh. Two people would say "I got first interrupt on X", one of them would quickly adjust and say "I'll get second" and you hope that's where the confusion ends. Might be better ways to deal with it, but I think it makes for an interesting mechanic to try to reproduce that.
Requiring DPS or tanks have to quickly target an ally and save them with an extra action button is a nice change of pace for them also.
edit: Thinking about it some more, you only have to assign the first shield in the 5 second charge up time, you have 10 seconds - until the first forcefield is expiring to assign the second forcefield and the third one is obviously whoever is left. This might be a bit too lenient, numbers could be adjusted to make it more challenging.
Thoughts? What do you think of the ability? What do you think of the idea of a voice comm required ability? Do you have a better way to do it, or something else entirely?
Let's hear it.