I'm super against any sort of imposed role restriction at the queue level. It will make the queues worse than they already are and will kill what is left of frontlines.

They need to re-design the rule set in order to make a balanced party more desirable. This was done well in secure, although they could definitely do an even better job.

They could modify the way party buffs works, for example right now you get a VIT bonus for having 1 tank, but there's never any reason to run with 2 tanks. They could give warrirors and paladin's a separate VIT bonus each. This would encourage 1 of each tank, and if you stack 6 BLMs 2 healers, you're going to have ~400 less hp which is fairly significant. They could extend this to DRG/MNK, a double STR bonus would make having a 4 melee group of WAR, MNK, DRG, PLD start to look viable. BRD/NIN would benefit if there's one of each. Even SMN/BLM could give a double INT bonus that encourages further diversification (and this buff could obviously be tweaked and balanced as necessary, ex: maybe the BLM buff gives 1/2 the current INT buff received, so having 1 SMN + 1 BLM would result in the usualy +3% party buff).

The beauty of this is that it won't impact queue times, you can still run whatever janky party setup you want, but there is an incentive to do something more balanced.

They also definitely need to add some extra objectives to slaughter to break up the fighting a bit and encourage 8v8's. One thing they could do is when the side nodes pop at markets/manors/temple, they could make it so that there is a barrier on each side of the node. You need 5 people to stand on the panel and the barrier on your side drops, your team gains +10 points (they can increase the total points needed), and then you can proceed to attack the node. This would encourage groups to actually split for the side nodes and allow some interesting 8v8 battles. Melee would be more useful here as a group with 6 BLMs might do well up top, but wouldn't do as well against an equally skilled more balanced party.