I do not think that will ever happen. They would not force you to play more then one type of class, let alone three, to play the MSQ. Its the lifeblood of the game, and they need to make it assessable to everyone from casual players to crafters. Hence why they are making MSQ Dungeons soloable.

I can see two things happening. The first is that they end up not involving Cyella and Unukalhai in whatever happens to the Void due to having too many roadblocks in the way. That would be a shame. The second, and what I would hope happens, is that they add a new series of side quests that involves Cyella and Unukalhai, once the MSQ is finished with the Void. I personally do not think we will see it ‘cured’ in the 6.x patches, so would rather see that happen or get started via side content that is more free to put up these roadblocks in the name of storytelling.