Great Idea!
My proposition: Everytime you get enchant arrow (except Enchant arrow +1, of course), you choose another type of arrow you can cast, beside the normal +x arrow. Maybe the old ones even upgrade with additional +1 enhancement bonuses (but nothing else).
En example:
AA1: You have +1 arrows
AA2: You have +2 arrows. You also can choose another type of arrow, like +1 flaming.
AA3: You have +3 arrows. You also can choose another type of arrow, like +1 dragon bane (or a +2 frost arrow). Your +1 flaming arrows become +2 flaming arrows (you can NOT turn them into, say, +1 flame frost arrows, but you could choose +1 flame frost as your special arrow instead of the dragon bane arrow)
AA4: You have +4 arrows. You choose a new type of arrow, like +2 flaming burst arrows. Your other arrows improve (+3 flaming arrows and +2 giant bane arrows)
AA5: You have +5 arrows. You choose a ney type of arrow, like +1 giant bane icy burst arrows. Your other arrows improve (+4 flaming, +3 giant bane, +3 flaming burst).
Of course, you cannot have flaming arrows at AA 1, or giant bane arrows at AA2, because an arrow has to be +1 to have any other magical ability.