My guess is that enemies will probably respawn frequently so no area will ever be tapped out by other players, or become unplayable just because you visited once. I think they will have something in place to give you reasons to visit certain areas several times if you feel so inclined to try adventuring there again.
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I could envision quickly-respawning "trash mobs" in wilderness areas and the outer areas of ruins and such, just to keep players engaged and on their toes, with tougher boss-type foes at the center of the ruins or other areas of interest that respawn slowly to prevent farming, especially if they drop or unlock named or unique loot.