D&D 5E How powerful would a Fighter subclass that added both STR and DEX to damage be?

[Fenris-77]

Are you certain this doesn't add to Strength instead?

No.

 

[Willie the Duck]

If the goal is simply to make a fighter who uses both Dex and Str-based weapons (say, the old AD&D fighter with greatsword and longbow), why not this:
"Aptly-named Subclass ability(level 3): when you spend an ASI to raise Strength or Dexterity by 2 points, it instead raises both of them by 2 points"
So if you assigned points to start with 16 in both, you can be maxed at L6 (and can spend the rest of your ASIs on both Sharpshooter and GWM, etc). If you were more like Str 16/Dex 10, well then your Dex gets up to a respectable 14 (decent enough for a fallback weapon) by the time you max Str.

 

[Kobold Avenger]

If the goal is simply to make a fighter who uses both Dex and Str-based weapons (say, the old AD&D fighter with greatsword and longbow), why not this:

It isn't, and the weapons, which I'm thinking about can be any melee weapon whether it be a Greatsword or a Rapier. It's about giving something to those who are both strong and dextrous.