My version of Firebolt would be...
1d10 as per 5e
OR
target terrain to ignite a flammable surface (not stone floors) or a brazier, torch etc. If a creature is standing it, Dex save or 1d4 damage. [snip]
There is already a cantrip that kind of does your latter description: Create Bonfire.
"You create a bonfire on the ground...any creature in the bonfire's space must succeed on Dex ST or take 1d8 fire damage. A creature must also make the ST when it moves onto the bonfire or ends it turn there. The bonfire ignites flammable objects that aren't being worn or carried"
I would be perfectly fine if Larian changed firebolt to have 2 uses: normal firebolt targeting an NPC or target the ground to work as ^
