Love your ideas here. I had the pitchfork in hand regarding the effects from cantrips, but your suggestion of allowing them to set a ground effect if targetted at the ground only is a nice comprimise between Larian DOS2 rules and 5e. Thats about as close to a DM allowing a request of a player as you can get. Just make sure the damage from ground effects can only happen once per round unless triggered by an extra oil splash etc. I don' like being incinerated by ground fire at low levels in a short span of time.
