The ability to add keen to a weapon would be a great stone of change addition. Design it such that players would still rather keep the crit feats rather than attune and perform the keen ritual on their main weapon stash. However, now the player has an interesting choice when they loot that one weapon in another type:
1) Do I try to trade this rapier of banishing for a scimitar I can use?
2) Do I have a feat to spare for just 1 weapon? I can still trade it later if I find a scimitar.
3) Do I bind it and gather the ingrediants for a keen ritual?
I like this idea because keen is a powerful bonus, but adds no new imbalanced weapons because the bonus is already available through feats (feats that will still be used). Also, players may be more inclined to keep that xbow of smiting or holy burst heavy mace rather than all rushing for the favored weapon types adding some diversity to the game. In general ranged combat would be a little more appealing for all classes.