Flamestrike
Legend
Just tweaking the numbers does not get us anywhere. We're just sliding our setting on a scale - no matter what we choose the feat will always be a trap for newbies on one hand, and abusable for the powergamer on the other. Simultaneously.
Dial back the feat so it can't be abused even with pretty intricate minmaxing and we have a feat noone will take. Dial it back up so newbies aren't likely to be trapped by it, and we are right back at -5/+10.
The core of the problem is the feat applying an effect to every attack the character makes. It becomes a force multiplier; a stackable bonus. 5th edition is very stingy in handing those out, and for good reason. The rest of the game isn't set up to handle individual attacks that do d10+15 damage (from regular human heroes, not big monsters).
To actually solve the problem we need to abandon the idea that the feat gives a damage bonus altogether.
PS. Nothing wrong with damage bonuses, and bonuses in general. But that assumes they're small: in the +1 to +3 range.
Compare to some rare and highly desirable magic weapon that deals +2d6 damage. Big-ass damage bonuses are in the game! The difference is that the game would never think of giving this out without DM approval (in the implicit form: unless the DM likes the item, it will never be found by the characters).
If this feat was something special, like perhaps the Svirfneblin racial feat, or the way Wild Surges depend on DM approval, then my case against the feat would be much weaker; and the argument "just ban it if you don't like it" would have actual merit.
But as a common standard feat made available through a package deal (opting into feats), to all characters, and already at level 1...?
Deplorable.
The feat expressly requires DM approval. Feat are expressly optional rules.
Not that you need it with rule zero but anyway.