
My idea was that once you've vanquished a certain area you would be given an option next time you enter that instance to have no enemies spawn. You could then lark around at your leisure maybe doing some cartography or just getting some screenshots, whatever. I believe a similar thing happens in Zen Daijun for Canthan characters who have completed 'Unwelcome Guest'.
To stop exploiters you'd make sure no chests spawn and obviously quests would be difficult without any monsters to beat. Although I'd allow quests where you need to talk to a NPC who always spawns (Like the Pass the Buck with all the ministers in Cantha for instance).
I'm not sure if this would be would be worth the Devs time and effort but I thought I'd put the idea out there anyway.