Ask and you shall receive;
You know what would be really awesome?
If when you log in after logging out at your bed, you can randomly get queued into a special Duty Finder group, hopefully seamlessly; you log in and you're in the instance, rather than getting the option to queue, since that would just take the excitement out of it. You and seven other completely random people just wake up in a nightmare together and have to beat Diabolos. Not as a serious event of course, more like what Odin and Behemoth are now (purely for vanity) and a "proper" Diabolos fight can come later like how we're meant to get a proper Odin fight at some point. Since it'd be a nightmare it would have to be quite scary, though I'm not sure how they could achieve that effectively... Dark Souls level punishment and if you die you get kicked out? Heck, rather than an instance like the Primals, a mini dungeon filled with jump scares and things like the Tonberry Stalkers in Wanderer's Palace? If you cannot get raised and you just get kicked out it'd be all the more scary if people don't get replaced... One by one you get picked off... Can you reach the end?
Would be all the more epic if they don't even mention it at all and people just stumble across the fight at random, no update notes or anything, you just log in one day and "WHAT THE HELL IS THIS OHMAHGAWD T_T". Even though I generally hate the Duty Finder, I'd actually find being thrown into something like this, purely at random, to be a lot of fun. They'd have to give it some tangible rewards though, otherwise people will just leave... Guess they could always just make it soloable, though that loses some of the fun... It'd make a pretty damn good trailer for a proper Diabolos fight though... Update 2.x secretly adds this and you get a sneak peak of Diabolos at the end if you beat it, then update 2.x+1 adds Diabolos...
They could even reuse the same system for events, like Halloween. Imaging doing the regular Halloween quest, logging out in your Inn, then logging on the next day and you're trapped in a Dynamis like version of Eorzea until you finish the real Halloween event... Fill it with various monsters from quests you've completed and actually make it nightmarish (they're all Lv.99), you can either run back to your bed and wake up, or try and finish the quest.
It's December and my mind has just reached Halloween, what's up with that?