Not sure yet, though areas of both types are penned to exist. There will definitely be areas where performing certain magics are easier, though the details are still being worked out . We'll have to see. Though a battlefield full of corpses is going to be good place for a Necromancer anyway...
I think we should also adapt MoM's system of "Nodes" for this need.
Basically, Nodes are centers of a certain types of "Influence". This influence changes the surrounding landscape depending on how the specific node is. Also, nodes can have multiple influence types.
For example: An area with a magma vent acts as a weak "Fire" node. As you approach it, you'll encounter things like salamanders and a fire spell like "Flamelash" which would normally cost 40 mana is reduced to 35 mana or have stronger effects. Similarly, water spells would cost more mana to cast and have weaker effects.
On a much more powerful node, such as a Demongate, which spreads "Dark" and "Death" influence, can affect things much farther away. From several mile away you may encounter vultures and nearby villages are either abandoned or housed by cultists/possessed. As you get closer, minor undead and imps can start popping up. Closer still will be things like Wights, Ghasts, Demons, etc. And at the center would be greater demons and undead, such as Liches and Molochs.
In addition to other "Elements", I'd like to add "Magic" and "Savagery".
The Magic Influence is a universal effect that combines with other influences. For example, a mountain which has moderate "Ice" influence can result in the spawning of generally mundane creatures such as bears, while one with both "Ice" and "Magic" can spawn things like Snow golems and Frost elementals depending on the intensity of the influence.
In casting, magic influence is a double edged sword. The magic in the air increases the effect of your spells, but the same cloud may decrease your chances of successful casting. If you're skilled enough, an area with high magic influence would greatly benefit your spells, but a miscast could be disastrous.
Savagery is another universal influence that affects monster spawns and types. "Anti-savagery" usually "radiates" from settlements, though it depends on the settlement. A small human settlement won't have as much as an effect as a city, whereas a barbarian settlement would barely affect it at all.
We can also have "Artificial" nodes, which may be temporary. For example, a city of wizards would radiate "Magic" influence due to the experiments, accidents, artifacts, etc. An old battlefield may start to radiate "Death" influence depending on the amount of deaths in the battle; and may start creating undead on its own with a high enough Magic influence - which may come as a side effect if there were a lot of powerful spells cast in there. Creatures that are strong enough may radiate their own influence or even make their own permanent nodes. You may even be able to cart your own "Temporary node source" behind your army to strengthen your spells or weaken enemy ones. So an army without a wizard may want to bring along an "Anti-magic" node or a node type opposite of the enemy spellcasters in order to hinder them.