It's important that attacks/spells that normally deal 0 damage on a miss, in this surface-change, deal 0 damage on a passed ST. Exactly because of the Concentration mechanic of keeping spells going. Keeping up concentration is a big part of spellcasting classes, and sources of guaranteed damage mess with this because they result in a concentration check.
Your "firebolt on the ground" example would deal a guaranteed 1-2 damage (prompting a concentration check) and then a possible additional concentration check each turn. This is much more powerful than a single "concentration check if the attack hits," and unduly punishes spellcasting classes.
Maaaaybe it'd be okay for leveled spells to cause automatic damage as a surface because you're still spending a valuable spell slot to do so. But at the very least cantrips should not.
I believe you mean direct attack ...
Since Shatter, Thunderwave, Burning Hands, ... those are if i dont forget any all AoE spells ... and they all have granted damage, since on succesfull Saving Throw they still give half damage.
But i get what are you talking about, you pracicaly dont even care about damage ... concentration is the point here.
Prehaps we should look a little deeper for this.
WHAT IF! And i admit im not quite sure how hard this would be to implement ...
Saving Throw for Concentration would not be triggered by recediving damage ... but by action (or hostile bonus action ofcourse) of your enemies?
I mean ...
If concentration saving throw would be triggered by action (or BA) ... you would get single concentration roll per spell/attack used against you ... no matter how many other rounds it will give you damage.
That way it would still have its uses to do AoE attacks, since you can hit more enemies and lets face it 2d4 is better than 1d8 ... yet spellcasters would not be punished.
Now when i think about it ... it would be easier to tie it to attack roll (whenever sucesfull attack roll is made against you, you have to roll concentration) ...
With spells like Magic Arrows, and Shatter would force it as part of their own code (like hit target for 1d4 AND force it to roll concentration if possible)
//Edit:
Or simplier way ...
Every time an "Effect" cause you to roll concentration, you get autosucess for next round from the same effect.
Aka: Burning AoE >> Concentration roll ... 2nd round >> Autosucess from this source
//2. Edit:
It just hit me ... when you say that cantrips should not give AoE ...
I suggested that spell should sacrifice single damage die to add AoE effect to its attack ... just as Chromatic Orb do now (maybe i expressed myself poorly again) ...
That automaticly exclude cantrips, since they only have single dice ... i thought its obvious that only spells that give 2dX and more could do that.
