Nice thread.
I agree with Mouseferatu about keeping a distinct feeling for weapons feats, so the actual problem I have with GWM and Sharpshooter are the fact that they are ostensibly for different weapon types and damage dies (d8 vs d10/d12/2d6), but largely give the same effect, -5 to hit for +10 damage. The plus 10 damage reduces the importance of the tradeoffs between those weapon types, proportionally speaking (d8 gets more % benefit than d10, d10 more than d12, etc). Which is the opposite of what it should do. The greataxe should get the biggest benefit from GWM, to my thinking.
What I would do is this; instead of -5 for +10 damage, I would say, trade either a -5 or one of your extra attacks for max damage or maybe double damage. That way the weapon's damage die is kept into account, you get up to a +11 max damage boost (for great swords) and +7 for arrows with sharpshooter. Or something along those lines. I like doubling the weapon's damage potential, I think that's fair.
For sharpshooter, to keep it distinct, I see it more like a sniper, who takes his time. So trade off one attack for double damage on a hit for this attack. Or maybe just make it an auto-crit. With great weapon master, I would make it do double damage dice in exchange for -5 to hit.
Static damage bonuses are boring, I agree. They reduce the relative importance and value of a character's weapon choice too. For example, Polearm Master combined with great weapon master is very overpowered I think, because you get one or two extra attacks, with reach, per round, with a d10 weapon but you can still use -5 to hit for +10 damage on each one. That makes it way better than both greatswords and greataxes. Plus it's still a single weapon, so you can find magic versions. Polearm master should not have given an off hand attack with strength mods for free either. Especially not when it can be combined with GWF so easily and so fortuitously.
A variant human fighter at level 5 can have three attacks at +1 to hit for 1d10/1d10/1d4 + 13 damage on each one. With reach. Plus an extra attack at +1 / 1d10 + 13 for an opportunity attack quite often. It's a blending machine.
Some of these feats were not thought out very well at all.