I like the idea of two branches (pardon the pun) of bow technology, one being crossbows for higher hit, only once per turn fire, and bows for not as much damage but a much better rate of fire.
To be fair, though, a crossbow IS a development from the bow, it's considerably more complex to build a crossbow than to build a longbow. Here is my idea for the new technology tree:
Archery
Unlocks: Early Crossbows (technology for research)
Unlocks: Bow Ammuntion (technology for research)
Unlocks: Longbows (technology for research)
Unlocks: Archery Range (Building, allows creation of bows and ammuntion?)
Longbows (technology)
Unlocks: Longbow (weapon)
Unlocks: Composite bows (technology for research)
Composite Bows (technology)
Unlocks: Composite Shortbow (stonger shortbow)
Unlocks: Composite Longbow (stronger longbow)
Unlocks: Volley Fire (technology for research)
Volley Fire (technology)
Unlocks: trained bowmen who all attack in a single volley (increases accuracy, ends bowmen turn)
Bow Ammunition (technology)
Unlocks: War Arrows (equipment, increases bow damage)
Unlocks: Advanced Bow Ammunition
Advanced Bow Ammunition (technology)
Unlocks: Fire arrows (adds fire damage to bows, damage bonus)
Unlocks: Pile Arrows (damage bonus/reduces effectiveness of heavy armor)
Early Crossbows (technology)
Unlocks: Light Crossbow (weapon)
Unlocks: Improved Crossbows (technology for research)
Unlocks: Crossbow Ammuntion (technology for research)
Improved Crossbows
Unlocks: Crossbow (weapon)
Unlocks: Heavy Crossbows (technology for research)
Advanced Crossbows (technology)
Unlocks: Heavy Crossbow (weapon)
Unlocks: Repeating Crossbow (clip fed, same rate of fire as a longbow, damage as a crossbow)
Unlocks: Ballista (Technology for research)
Ballista
Unlocks: Ballista (seige weapon, giant crossbow that basically shoots spears, good for dragons)
Crossbow Ammuntion (technology)
Unlocks: Barbed crossbow bolts (increses damage)
Advanced Crossbow Ammuntion (technology)
Unlocks: Crossbow quarrels (increases damage, accuracy)