This might actually be the fastest way to get new biom content out. Sometimes its better to start over and redesign something than try to get something old working that you just threw together when you didn't really know what your were doing.

As you pointed out the Mistlands looked awesome but from a programmers point of view it might have turned out to be an incredibly bad biom design. So instead of wasting time trying to get the monster AI to work in the old Mistlands they are going to change the Mistlands so the existing AI works in it.

IG will probably reuse the old terrain assets so it will look very similare to the old biom. Or it might be something completely new that was created to work with what they already have in the game.,