I know this is partly a lack of creativity on my part, but i struggle to think of any dungeon-appropriate fight mechanics that i would find "interesting and fun" the 20th time i did them(let alone the 100th). The issue is that dungeons need to be designed so you can run them endlessly with a fully interchangeable party. Sleeping the guard dogs to put the enemy in a less alert state would be an interesting mech. It would also suck for every group that didnt have a BLM/WHM to cast the spell. Enemies that tried to run away and alert/spawn more friends to help would be interesting. Not every group composition can bind and/or assure the mob dies fast enough. In either case, after 5-10 runs it would just be another pattern to follow and not all that fun or engaging. Raids/trials give the devs a little more flexibility in fight design, and indeed those are where the more interesting mechs have shown up recently(and a few of the recent solo duties as well).