Well, I would give archers the possibility to loom arrows, as for example infantry can loom its shield or cavalry can loom its horses, but I would say limit it on 2 more shots per loom level. You would have large bags of arrows (+2), really large bags of arrows (+4) and supersize bags of arrows (+6), which would hopefully increase motivation to heirloom arrows.
I don't think we should restrict heirlooming to certain items, as cRPG is about variety and choice.
But the more I think about it, the more I would support increasing heirloom bonuses accordingly to the slots an item takes.
Because in theory time is endless, and so are loom points.
Now let's compare a pikeman to a 1hd-cavalry-horse-archer-hybrid.
The pikeman will most likely only use his pike, because there are no other items that make sense in combination with an unsheathable, two handed 3-slot weapon, so he could heirloom:
- Pike
- Armour
- Helmet
- Gloves
- Boots
Whereas the hybrid can heirloom
- Bow
- Arrows
- Shield
- Sabre
- Armour
- Helmet
- Gloves
- Boots
- Horse
Which, if you think about it correctly, on long term strongly favours the second build.
So I think it would not be unfair to give a pike +2 damage and +1 speed per heirloom level, because playing a pikeman is currently the least rewarding class in cRPG.