An experimental area that tries to distance itself from scripted gameplay would be exciting to see. I'd like it to fit around normal class design as well, so no additions like duty actions or lost actions. Combat would be similar to what we're used to except that enemies would be less predictable and also react to what happens in the fight. A boss enemy could have some hidden attack resources and spend them in different ways each time it's encountered and if the party enters a state with low HP the boss could become more willing to spend resources on damage. That has the potential to create more demanding healing situations without resorting to instant death mechanics. Think of mini soft enrages that happen through out the fight if the right conditions are met. Tanks could also be given more opportunities to defend the party by being able to interfere with enemy attacks through their own abilities (Low Blow, Interject, etc). The effects of these abilities should be tuned to be less binary. Stun could be turned into a slow effect or modify the range of AoE attacks instead of completely incapacitating something.

Such an area would be a great place to test ideas for mid level difficulty that the game is missing and offer something different from scripted Extreme and Savage fights.