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.
For the casual player, it is a perfectly ok option. For a powergamer, almost any option (from actor feat assassins, to grapple-prone-locking, to armor of agathys/weavy armor master/paladocks, to cite a few) can become a force multiplier in unexpected ways.
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..
See, this somewhat prevents the discussion from taking a more constructive direction. I acknowledge the feat is a problem for several tables, but I see it is not a problem in several other tables, that probably have a somewhat different playing style or difficulty level set by how their DM is building up encounters or whatever. I can't seem to notice you acknowledging this other side at all. Instead of trying to build some option that would satisfy yourself, you seem to spend a lot of time just trying to prove everybody else who has a different opinion from you is wrong. And then, the discussion derails, it goes from trying to work out interesting options for the feat to a heated discussion about whose opinion is right, which is what we are wasting time doing right now.
You point, for instance, that the game is not set up to handle individual attacks that do big damage. Isn't it really? Can't the rogue sneak attack all day long for up to 11d6 +5, only requiring an ally adjacent to the enemy? Can't this be effectively combo-ed up by the group consistently providing this same rogue with opportunity attacks? Isn't this ability of the rogue also an enabler for a combo?
You seem to bee too worried by this high damage per attack. There is at least one build which does not require feats or multiclassing or any other optional rule at all, that can be achieved with standard array, and allows for attacks dealing 3d6 + 10 damage basically all day long, without even having any penalty to hit. Quite similar to your d10 + 15, isn't it? And it is not dependent on the right circumstances (advantage, no high-AC enemies) or group cooperation (bless, bardic inspiration). Is it broken too?
Now, one thing I am trying to figure out is, what exactly do you (the general you), who does not like the damage boosting potential of GWM feat, expect a feat for great weapon wielders to do? It was pointed out earlier in the thread that there is no option for the S&B to up their damage to levels comparable to SS or GWM. That is true. But is that bad? I mean, if somebody goes for big melee weapons or archery instead of S&B, sacrificing the +2 bonus to AC, isn't that person already giving the message they want offense over defense? With feats, which usually make PCs more narrow in focus than simply taking ABIs, wasn't it expected that feats to support for those two styles would most likely just go for damage boosting?
Just to address the comments on S&B becoming somewhat weak, undesirable or overshadowed because of GWM, I don't see that happening. S&B's have other options, that just do different things. In my opinion, shield mastery leaves nothing to be desired when compared to GWM, and in some groups/playstyles/campaigns it is simply strictly better. S&B builds based on DEX can also take defensive duelist, which is pretty nice too. Neither of those feats boosts damage directly (although shield mastery actually allows for nasty damage-boosting team combos), but they are, nonetheless, competitive and valuable weapons on the powergamer arsenal.
There are several other options. For instance, a mix of warcaster, sentinel and either magical adept or just being an eldritch knight can do wonders for damage boosting potential, while even augmenting frontline stickiness. This may seem a long string of feats, but a variant human fighter can do the trick already at level 6, or 4 if going eldritch knight. In fact, sword and board eldritch knights can have ridiculously high damage output while keeping a very powerful defense by utilizing tactics to exploit their cantrip-charged opportunity attack, having a high base AC, and being able to top that up with Shield spell when the need arises. Is this ok, or does it need some nerfing too?
To each one its own. What I know is that I would not even bother to play as a warrior with a great weapon in a featless game, were I to let my powergamer side decide char-building (which actually is not usually the case, as I am very fond of (half)orc sorcerers, warlocks and rogues), as I don't think the base damage boost from a rapier/longsword to a greatsword is enough to make up for the AC loss. Worse still, it actually closes the door for a dex build, which has nice perks. SS is another beast altogether, but it has more to do with the obvious benefits of staying out of harm's way to begin with.
Now, finally trying to go back to track, I really think that whatever option is to be tinkered for great weapon mastery, it would be advisable to accept that for it to be desirable by someone going great weapon (and thus pursuing offense), it will probably go the way of damage boosting somehow. Nonetheless, any flat bonuses without hindrances are a bad idea, as they will seem like a boring option, and worse still, a feat tax for those who want to pursue this combat style.
Making the feat allow for a single attack dealing the full combined damage of all the attacks a warrior would have plus some extra to make up for the "all in or nothing" aspect, as already pointed out, is an interesting idea, as it would keep the high-risk/high-reward concept still valid. It would not be useful for mass gnoll killing, though, which points to limitations to its usefulness. It would also make the feat a not-starter-level deal, as it would depend on the extra attack feature. I think some people will like this perk while others will not be so pleased.
Another option would be to allow for exploding dice (rerolling maximum numbers for damage). I know some die-hard Earthdawn fans who would just love that. To invoke the high-risk/high-reward element again, this should come with a drawback, maybe letting the feat reduce base weapon dice, or give some to-hit penalty, or even some AC penalty while the warrior pulls this trick. Of course, when I think about all the options I just gave, I can already see they can be somehow exploited, in different ways from the original feat, of course, which leaves me wondering, is there really any option that would still incur into a legitimately meaningful choice, but still exploitation-free?
