There is a feat in D&D 3.5e called Monkey Grip. I am considering this 5e feat inspired by that:
Wielding a weapon made for a creature one size larger than you doesn’t impose disadvantage on your attacks.
This means, assuming you are Medium and you wield a Large weapon, the base damage die of the weapon is doubled. At best—a Large greatsword or maul—this adds +2d6 damage, or on average +7, to each attack.
Great Weapon Master offers +10 damage at a −5 attack penalty—but you can also choose not to use it if you need the accuracy, and Great Weapon Master also offers the option for a bonus-action attack on a crit or kill. I strongly suspect that a +3 relative bonus in damage is not (remotely) worth a −5 penalty to attack, and I have doubts that the bonus-action attack, as limited as its triggers are, is going to make up the difference. Is this version of Monkey Grip clearly superior to Great Weapon Master? Is there any case in which Great Weapon Mastery would be the optimal choice when this Monkey Grip is available?
The comparison with Sharpshooter, it seems to me, is a little better—the same −5 attack for +10 damage, but the other effects of Sharpshooter seem far more valuable. How does that stack up?
The other consideration is that Monkey Grip and Great Weapon Master could be combined, for a possible +17 damage—unprecedented so far as I know, since most of the direct combat feats are incompatible with one another, unless there’s some way to make a “melee attack” with a “ranged weapon” in order to qualify for both Great Weapon Mastery and Sharpshooter. Still, we could just add a clause to Monkey Grip barring it from being used in combination with Great Weapon Mastery, if necessary, as I suspect it is.
The 3.5e Monkey Grip feat came with a −2 penalty to attack rolls. In that system, that penalty was not worth the benefit, but the benefit was only +1 damage, on average, not up to +7 (unless you really worked at it, but that took much more than just picking the right type of weapon). And, of course, 3.5e math and 5e math are quite different. So I have, for the initial version of the feat, left that out—but the comparison to Great Weapon Mastery leads me to suspect that Monkey Grip needs something. Is a −2 penalty the answer? Comparing Great Weapon Mastery and Monkey Grip, you’d be looking at a −3 relative attack penalty for a +3 relative damage bonus—is that better-balanced? Does it expand the situations in which Great Weapon Mastery is the optimal choice? Does it leave other cases where Monkey Grip is the optimal choice?
Most 5e feats do more than one thing, too. Great Weapon Mastery has the bonus-action attack, Sharpshooter mitigates the difficulties of long range and/or cover. Monkey Grip should probably have something too—I’m kind of leaning towards an Intimidate-based effect, since the whole concept of Monkey Grip is the badass image of someone with a huge freakin’ sword—be nice to see that image have mechanical effect. But I have left that out, too, on the basis that Monkey Grip already looks too good compared to Great Weapon Mastery and GWM’s bonus-action attack or Sharpshooter’s range/cover mitigation might make up the difference. So it would be nice if answers also addressed how large a consideration the bonus-action attack or range/cover mitigation is in determining whether or not Monkey Grip is balanced—if, for example, the lack of an add-on feature makes the difference between the −2 penalty being “enough” for balance, I would want to know that.