When playing PC which uses archery more or less for the entire game, I always get annoyed by the large variation of arrow and/or quarrel parameters which prevent them from stacking.
It's not uncommon to gather 4-5 groups of 2-3 or 4 arrows/quarrels of *foo* slaying that differ by only a single point of to-hit or to-damage and subsequently do not stack within one useful quiver but rather have to be reloaded one after another.

In one of my recent games, I went on a longer campaign in BDC. Lots of ancient dragons and wyrms require lots of dragon slaying ammo, so here's a sample of my inventory just before taking Sharad-Waador's quest:

bundle of 2 uncursed arrows of dragon slaying (+3, 2d6+2)
bundle of 5 uncursed arrows of dragon slaying (+3, 2d6+4)
bundle of 3 uncursed arrows of dragon slaying (+2, 2d6+3)
bundle of 2 uncursed arrows of dragon slaying (+4, 2d6+4)
bundle of 11 uncursed arrows of dragon slaying (+3, 2d6+3)

I wouldn't mind if all of these could be stacked, even at the cost of losing a point or two points of to-hit and to-damage (rounding down to the lowest value).
Perhaps give the option to merge the different arrows of *same* slaying, averaging the damage and hit modifiers?
What about scrolls of increase melee accuracy/damage - could they not be made to work on projectiles similarly as on standard weapons?
Obviously arrows made of different materials should be excluded from stackability, or perhaps suffer a penalty of converting to normal materials to be able to stack with the rest?

Overall, it would streamline the inventory quite a bit and remove an annoying element from the game at next to no cost in balance, since you still have to find the arrows in the first place.
Perhaps make this an Archery skill dependent ability? I'm just doing a lot of guess work here, but the original idea still stands.