So, in conclusion, what's wrong with the NW of Unearthed Arcana is the lack of an automatic damage progression.
Starting at d6 isn't actually so horrible it can't stay, IF it gets automatically upgraded with character level (much like a cantrip):
Level 1: d6
Level 6: d8
Level 12: d10
Level 18: 2d6
The Greatweapon Fighting feat is amended thusly (add this text to the end of the feat description):
If you have a natural weapon, its damage die is upgraded a number of steps decided by your character level.
Less than 6 levels: one step (a d6 becomes a d8)
6-11 levels: two steps (a d8 becomes a d12)
12 or more levels: three steps (a d10 becomes 2d8; 2d6 becomes 2d12)
This concludes my analysis and suggested fix for the UA natural weapon debacle.
Zapp
Heh, debacle. I love the dramatics.
Another one in this thread I love:
no high level hero will choose to make half damage against most level-appropriate creatures.
Since you specifically invited me to read this thread, I'm going on a different tangent.
You want natural weapons to scale, so somebody can choose to be a natural weapon specialist, and focus on that aspect. Well, OK, that's fine. They can do that as a pugilist too.
In D&D and other genres, minotaurs are often depicted using other weapons, commonly a battle axe. Why? Because as good as there natural weapons might be, real weapons are better. They are better than the natural weapons of other minotaurs, and also better than their own natural weapons. It makes sense that their horns don't keep up with steel weapons, otherwise there would be no need for
any of them to use them.
Having said that, I certainly don't object to the idea that there might be some that want to use their natural abilities and build them to their pinnacle. I'm still not sure I'd agree that it should keep pace with actual weapons, but something in the ballpark would be OK. Regardless, I don't think it should necessarily apply to
all characters of that race. The main thing that you're lacking in your analysis is the fact that there is an inherent advantage to always being armed. Whether that applies to your game or not I don't know. But considering the game world as a world, where there are many circumstances where you might be disarmed, or that bearing arms is inappropriate and not allowed, it does provide a real advantage.
So the only thing I would change is that I think your proposal should be a feat, rather than something that every minotaur or creature with natural weapons gain automatically.
I also think that your amendment to Great Weapon Feat is irrelevant, since I can't think of any natural weapons that would be categorized as heavy weapons.
Second, if there is a world where minotaurs exist (or other intelligent races with natural weapons, are civilized in that they have an economy, mine, forge, develop armor and weapons (or adopt those from a society that does), and there is magic and magic items, I certainly think there would be magical horn sheaths.
My biggest issue with all of these types of races is that just dumping them all into the core world, without addressing things like cultures, histories, the fact that they would most likely have developed and evolved quite differently from humans, is never addressed. They end up as humans with horns, or whatever. Otherwise they don't differ at all.