I'm not sure I get how that would work. Have you played much Mobius MP? There's quite a lot of depth to it with the different elements, job roles, and especially in regards to turn order. Attacker needs to break the yellow gauge, breaker for the red gauge, attacker to nuke after (but keeping enough orbs and actions to not completely whiff during a break) plus juggling buffs/debuffs/defense drives from supports and defenders and making sure someone has enough turns to generate orbs for everyone on the last turn. And the virtually unlimited combinations of decks and specialized weapons. I don't know how Exvius would be nearly as interesting with such limited customization if you're only playing one character in a party doing one action per turn.
- Raid boss with immense HP and some very strict mechanics (basically the trial bosses we already had but with more HP for balancing purposes).
- 6 members party (or more, maybe 12 with each player controlling 2 characters) where every player controls their own actions.
- No possibility of using repeated characters, therefore no chain cheesing.
- If the party dies you can try again, but the fight resets. If you quit you can't play again with the same friend, unless he uses a different character.
Basically it could work as a convential trial boss, but with the possibility of having a more diverse and complete composition as each player could contribute with their BiS character for the respective position. Right now only the hugest whales have a complete BiS character for every position (healer/tank/support/mage/dps).
Due to this, the boss would need much more HP as well as more strict mechanics like healing himself, having immunity to phy/mag damage on some turns, having immunity to some elements in other turns etc. With this, elemental chaining (even with different character) wouldn't be so broken as it wouldn't work all the time. Plus, this could be handled with characters that can debuff elements giving more importance to these types of moves in the place of spamming the same damaging move over and over.
Something in those lines in a mode with a different NRG consuption (could be orbs) and interesting farmable rewards (1% or 5% TM moogles for every single character in the game, elemental weapons, immunity accesories/gear, crysts and stat enhancing pots) would be very good as a challenge to whale players with their complete characters for every position and not so whale players with that one good PHY/MAG damage dealer or TANK/SUPPORT.
It would be challenging, rewarding and inclusive as you would only need at least one/two well equipped character. Plus, it would give you a reason to play the game besides TM farming/Mog King grinding, it would give you the incentive to be part of a community and it would be updated every single week with the very same rewards, so there would be no reason for you to rush or grind it, it would always be there and it would always give you the type of reward you could get elsewhere doing your TM farming/Arena/Cryst Vortex/Mog King events.
That's my dream multiplayer mode. Something very Record Keeper (maybe not so Mobius) inspired, but converted to our reality, our mechanics.