The point of stats adding X-X damage to spells is to be best for rapid fire spells while bigger spells get more of a bonus from +X% damage type.
Those X-X added damage does scale with any % increases to the damage type of the added type.

You are correct that flat damage bonus best affects rapid skills, but u are wrong saying that big spells benefit more from percentage. Percentage damage affects equally all spells.
Simple example. Fireball: 100 dmg, Tear of Etheliel - 200 dmg. As for now Tear is twice stronger.
We wear ring +100% spell dmg.Now: 200 dmg vs 400 dmg. Tear is still only twice stronger, It didnt become more powerful in relation to Fireball. So rapid skills have a bit unfair advantage.

Of course % increase damage affects all spells equally.
But between the 2 choices (% increase or +x-x damage) bigger slower spells benefit more from the % increase while faster smaller spells benefit more from the added damage.
The numbers of these stats need to be balanced correctly of course, but they should affect all spells equally.
Otherwise we end up with 1 stat being superior in every situation. That's not interesting for an ARPG.

As for rapid spells having an advantage.
Maybe, but not because the numbers don't scale.
Maybe they don't, I haven't done the math of how much damage each spell does, so I don't know.
To cast multiple rapid spells to get the same damage of 1 big spell with a longer cooldown you need to do more casting and more just standing there. That's a lot less mobility.

"Bigger slower spells benefit more from the %" - absolutely correct.
"while faster smaller spells benefit more from the added damage" - and here is the problem: no matter how much %dmg you add, they will be just as powerful compared to big ones, but the more flat dmg you get the more superior they become compred to big. Math is simple - dps is greater. Mobility and other factors have no influence since only damage changed.
To put it simple - there is no scenario where big skills have an advantage over small ones, but reverse is easy to achieve.

"There is no scenario where big skills have an advantage over small ones, but reverse is easy to achieve"

Simple scenario:
You're in a fight where you have to dodge a lot and don't get a chance to cast very often.

In this basic scenario bigger spells that either have a longer cooldown or cost more have an advantage as you cant stand there casting a lot of small spells as you need to be dodging again after you finish a single cast.

Longer cooldown spells also have their place as damage cooldowns in between spamming smaller spells.

IMO, what it comes down to is, if the flat damage stats and the % damage stats are balanced well, then there is no need for this scaling.
If they aren't balanced well, then they just have to fix the balance.

You don't quite get what i am talking about. Of course big spells often more useful in combat but i am talking about gear stats influence on spells. It's easy to build your character so rapid skills dps would outshine big spells, but no matter what stats you have, big spells cant get better dps compared to rapid.

Then the balance of the different stats and the spells themselves need to be looked at.
The added damage stat numbers shouldn't scale. That's a bad idea.
If we were to scale them, then flat added damage would become the best stat in every situation/build. That's super boring and the opposite of what an ARPG should strive for.

So in conclusion, if you feel the bigger spells aren't strong enough compared to their rapid fire counterparts. Then give specific numbers to what spells are underperforming, how they are underperforming and ask that they get buffed instead.

I'm done with this thread.
I have understood everything you have written despite your language insisting I didn't.
Your suggestion should never happen.
I have explained why your suggestion should never happen.
If you don't understand, that's fine, but it's not worth any more time investment to me.

Sry man, seems like I have not explained my point clear enough.
Your quote - "Then the balance of the different stats and the spells themselves need to be looked at." - that is what we both want. I have edited main post to clarify things.