I'd like to test version where cantrips only put single turn worth of surface, and you don't get free prone from the frostbolt. I *like* the utility, throwing frostbolt at fire makes sense regardless of what PnP purists might think.
Also, damaging surfaces in general need changing, you shouldn't get more damage by running for 6 seconds than for standing 6 seconds in it
IMO half move in fire or more = single instance of damage should be fine, so you can run thru any tiny fire surface without any damage but not thru burning building
