D&D 5E - Versatile Fighting Style

As a Fighting Style there should be no bonus action cost since others don't use it.

Otherwise, this is superior to Defense, which requires armor and grants the +1 AC bump. With this you would not need armor and could get +1 to attack rolls when you needed that instead.

Maybe a style where you can give versatile weapons when wielding with two hands either the finesse or heavy property?

This would benefit the wielder by allowing them:
1. to use DEX instead of STR with weapons like a longsword or battleaxe, and
2. to benefit from GWM (which requires "heavy" weapons for part of it) with a longsword or battleaxe.

I would write it up as:

Versatile Fighting
When you are wielding a versatile weapon with two hands and take the Attack action, you can choose either of the following benefits:
1. You gain a +1 attack bonus and the weapon gains the finesse property.
2. You gain a +1 damage bonus and the weapon gains the heavy property.
3. (OPTIONAL?) You gain a +1 bonus to your armor class on the next melee attack made against you before the start of your next turn.

The +1 attack bonus is inferior to Archery's +2, and the +1 damage bonus is inferior to Dueling's +2 but you have the choice from round to round. It also allows weapons such as the battleaxe and longsword to gain properties that can be useful in other ways.

I think that would be a good balance personally. I added option #3 if you still want an AC boosting option (you could probably even bump it to a +2 bonus to AC...), which is reasonable and in line with your OP.

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