We know from the Twitter that you will be able to use the magic to raise volcanoes.   Its logical that high end magic may be able to create impassible terrain.  Hypothesis suggests that high end earth spells could create impassible mountainous terrain, which could be in turn crossed by units using some sort of "earth movement" spell.  Water similarly likely has high end water expansion spells.  Able to create rivers, or on high end create sections of ocean inland to take advantage of the water supremecy likely owned by the users.

You'd think 'raise/lower' and 'change climate' would be all that's necessary; you can't make a river just by dropping a bunch of water unless the terrain is cooperative.