Brick walls can be destroyed, so on some maps you could technically destroy all the walls (though realistically you probably wouldn't).
Pretty much anything wooden can be lit on fire, sometimes explosively. Fires can spread, and can be put out with water.
There are a wide variety of liquids that can form puddles, either spilled from barrels or from thrown bottles. Some of those are flammable or hazardous, and can also be washed away or cleaned with water magic.
Water can be frozen, ice can be melted. There's a water spell that lets you create deep water that would need to be swam through or flown others.
Unfortunately there's no electric magic in the initial release but that's the next element I plan on implementing, which will definitely involve electrifying water.
As to how much it will come into play, a lot of that depends on what spells you have and how you use them, but I think the most likely to happen no matter what is causing incidental fires with fire magic.