Maybe the hardest of the game is actually the oldest content (as far as dungeons are concerned). Maybe the devs focused too hard on bosses to provide any challenge to that content.

I used to play a different game where you would try pulling wall-to-wall and wipe long before you reach the boss (then again, in my old game, gear counts for much less than in FF14, and that's what I liked most about it) but trash is still light on mechanics. But I digress.

Maybe it would sound radical to you but removing the mobs from dungeons may be a viable option that won't affect how hard content is. Now I am not sure about how well-liked a trash-free dungeon would be, but it would work only with the right story premise. Possible premises for trash-free dungeons include:

1) Bosses being destitute or depleted logistically
2) A spell has been cast on it beforehand to whose effects bosses (and possibly adds) are immune but would-be trash isn't
3) Bosses seem to consider mobs useless against the party
4) Bosses hold back mobs until the party reaches a point deemed strategically important enough for a boss fight, thereby lulling the party in a false sense of security
5) Bosses being the last survivors of a catastrophe that befell the area
6) A distraction pulls would-be trash away (much like in Castrum Meridianum/Pretorium but more effective as a distraction)