What I'd love to see in an MMO is dynamic terrain. For example if you get a lot of rain in Thanalan part of it will flood and new rivers will appear for a while. Then the water drains away and it goes back to normal. Same with snow, have it turn the ground white. Strong winds, where are my fallen trees?! Why does Thanalan lack sand dunes if we get sand storms so often, why don't those dunes move? There's a volcano north of La Noscea, I want to see it erupt every earth year or so and change a bit of the terrain in northern La Noscea!

Maybe technology can't yet achieve this without it being too demanding on the system, I don't know.