A pretty simple method for improving them would be to reduce the initiative penalty across the board. They just seem so slow compared to everything else it's not worth giving to heroes, and as mentioned their damage gets poor quickly when enemies start fielding better armor.
Adding armor piercing to all of the bows or perhaps just the crossbows would work too. The crossbow ability as it stands is just rubbish. It's too situational and doesn't even make sense. Armor piercing is exactly what crossbows and the better longbows were all about though, historically.
A 50%/100% AP for crossbows and a 0%/25%/50% for bows (upgraded to 0%/33%/66% for the Archer trait versions) seems a good place to start, to see how it works out balance wise.
Those should all be very simple XML changes. Balancing out the existing unique champion's bows would be slightly more challenging, but still not too bad.
It's important to be careful to not make them TOO powerful though, as the whole point is you can deal damage without fear of retaliation. I know I find archer monsters and bandits to be most annoying in the early/mid game, before my spellcasting really comes into its own. That tells me they're not too far out of whack.
