All questions of flavor aside, how balanced do you think this feat is compared to Great Weapon Master and Sharpshooter?
Adept Weapon User (AWU)
- Before you make a melee attack using the Attack action with either a weapon that does not have the heavy property that you are proficient with, an unarmed strike, or an improvised weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage.
- When you score a critical hit with a melee attack or reduce a creature to 0 hit points with one, you have advantage on the next attack you make until the end of your next turn.
The idea behind the design is simply to give all martial classes access to a powerattack feat and to bring dual wielding, one-handed weapons and monks DPR-wise in line with two-handed and ranged weapons.
I deliberately chose to make the power attack only applicable to the Attack action, because otherwise Two-Weapon Fighting and Flurry of Blows would become too strong. Furthermore, I wanted to avoid making two-handed weapons strictly worse than sword and board (or worse, polearm master spear and shield), and so this powerattack cannot be used with opportunity attacks, the Ready action, bonus action attacks, or attacks from magic items (e.g. Scimitar of Speed, Dancing Sword). Additionally, the secondary feature of the feat is worse than that of Great Weapon Master. I also avoided clogging the bonus action, because monks and dual wielders already use this.
I have made some rough DPR calculations, but this is anything but rigorous. The biggest hurdle in a comparison is probably the fact that a player can choose to not use powerattack if the target's AC is too high (or its HP too low). One would need to have a distribution of AC targets and weight the values for each AC accordingly. To make this simpler, I have assumed a baseline hit chance of 65%, which is reduced to 40% when power attacking. Comparing one hit of a two-handed fighter (+4 STR, Defense Fighting Style), a GWM fighter (+3 STR, Defense Fighting Style) to a baseline sword-and-board fighter (+4 STR and Dueling) and one with the new feat (+3 STR and Dueling):
2H-base: (2d6 + 4) * 0.65 = 7.150 2H-GWM: (2d6 + 13) * 0.35 = 7.000 1H-base: (1d8 + 6) * 0.65 = 6.825 1h-AWU: (1d8 + 15) * 0.35 = 6.825 (this is a coincidence)
Note that these numbers do not reflect the powerattacker's option to not powerattack.
When the characters are high enough to get a +1 weapon and max out their STR:
2H-base: (2d6 + 6) * 0.70 = 9.100 2H-GWM: (2d6 + 16) * 0.45 = 10.350 1H-base: (1d8 + 8) * 0.70 = 8.750 1h-AWU: (1d8 + 18) * 0.45 = 10.125
Essentially, in this comparison a AWU-Fighter does slightly less damage than a GWM-Fighter per hit and has no additional chance for a bonus attack, but has +1 AC. Dual wielding is harder to calculate, because the number of mainhand attacks also plays a role.
The only loophole I can find is the Path of the Beast Barbarian's Claw attack, but even that is not excessively strong, in my opinion. Another downside of the current design is that the secondary effect is fairly weak for dual-wielding or sword and board barbarians.
Technically speaking it is possible to use both AWU and SS together when using a non-heavy ranged weapon as improvised weapon, similar to GWM and SS with a heavy ranged weapon (at least for a certain interpretation of the rules). I do not see this as a problem.