First, I know I may get a ton of negative reputation. But I need to give this suggestion anyway.
Second, a bit of background: With the exception of two weapons (Shuriken and Khopesh), every single exotic weapon need proficiency to get something cool done. B-swords and D-axes give glancing blows if single-wielding or using a shield. Repeating Light/heavy crossbow send 3-shot volleys. Great crossbow have the knockback on a natural 20.
Khopesh on PnP is a longsword with exceptional vertigo+2. This is too weak to waste a feat on this game. I'm not advocating for the khopesh to become what the B-sword was some years ago. But I think the khopesh can fit the DPS role as the same time it gets back into the power range of the other weapons.
The first thing that need to be done: We need to give khopesh something that can only be used if the character have the feat. So, without any feats, KHopesh is basically a longsword. No change. 1d8, 19-20/x2. No exceptional vertigo, no x3 multiplier, no nothing. This should solve the problem of people using non-proficient khopeshes because it's too good to the point that the -4 to-hit isn't a big deal.
If you get the proficiency feat, khopesh get the Bow of Sinew's Elasticity enchantment. So, on a natural roll of 19 or 20 (the weapon normal crit range), the crit multiplier get up to x3. So, while you do not possess Improved Critical, the new khopesh works exactly like it is now.
But why elasticity? Because even if you get Improved Critical, only 19-20 get the x3 multiplier. 17-18 get the base x2, which brings the weapon more on line with the B-sword and D-axe. It gets a crit power simmilar to scimitars, but with a higher base damage (and with the epic weapon damage change Eladrin advertised, would be even better later with 2d8 greenteels), it is more DPS than a scimitar. And still the best one-hander for TWF.
That would leave shuriken as the only exotic weapon that have nothing special on the feat. Maybe they can add shuriken expertise as an effect of the proficiency feat...