Basically, you can run through fire as much as you want, but when entering the fire or starting your turn there, you make a dex saving throw, and take 1d4 on a failed one, or none if you pass the save.
That way, they are in line with normal spell effects and everything is fine. Since, lets be honest, 1d4 damage per turn hardly matters, the real problem is UNAVOIDABLE damage, even if it is miniscule, since it triggers concentration saves
It doesn't seem to be in line in my opinion because players targeted by a fireball only make 1 saving throw.
If there are surface for a few turns after someone cast the spell : it is additional saving throw each turns, eventually additionnal damages each turns, and additional chances for your concentration to be broken each turns.
Unavoidable damages are worse of course but so many ST does not seem to be a good idea to me. Concentration broken too often is something I read a lot and one of the main reason is surfaces (from spells but not only).
Don't get me wrong I also like the idea of surfaces... but if it's subtle enough not to be too intrusive on the gameplay.
