Honestly, what truly needs to be done is these dungeons need to be redone so that they're more in line with the dungeons we have now. They're just letting it sit there, becoming more and more and more estranged from the rest of the content in the game.
Either redo the dungeon and quests surrounding it entirely, or make it a single player instance of some kind. The Ultima Weapon can remain in the game as a standard, end of launch trial that we've come to expect.

Main Scenario Roulette can then be changed to what it should be... a roulette for all of the main scenario dungeons and trials in the game. Any dungeon that you're required to go into as part of the main story is included, as well as any trial that's as part of the main story. Since every dungeon is 1 tank, 1 healer and 2 DPS, it can pair two groups together for any 8-man trial.