FormerlyHemlock
Hero
I was just messing around with this mechanic, and my solution was this:
Longbow: 1d4 + str modifier + dex modifier damage
I feel like this adequately represents what draw power gives you: a weaker PC would be able to use the bow, but not very well, and weaker PCs would be better off with a short bow.
On the opposite end, it maxes out at 12.5 average damage, which pulls well ahead of the heavy crossbow (10.5), rewarding its heavy stat investment cost. However, a player could achieve an equivalent value to the original longbow with relatively minor stat investment in strength (1d4+2 ~= 1d8).
This is also a very cool approach.