They technically aready do this. Its content that gets slowly released over time, and they've been doing all that since the begining, or at least HW.

The Wandering Minstrel
The Bingcoils
Diadem
Palace of the Dead
Alexander
Eureka
Heaven on High
Omega
The Ruby Sapphore and Emerald and Diamond Weapons
The Bozjan Front & Zandor.

Now if you mean changing the open fields encounters....thst wouldnt work as well in an MMO since there are vastly different character levels and progression that mY ne running around.