So there are a few items that have no effect when heirloomed, for example ladders or torches. (Yes, torches do give stats, but why waste a heirloom for a better torch? They're not supposed for that?) Thus, here are some ideas for uncommon items when heirloomed.
I don't think this will be too hard to code, will it?
Uncommon weapons heirlooms:
Torch - Increase lightning effect on heirloom.
Ladders - Increase healthpoints, maybe armor and/or size on heirloom.
(Construction site - Less materials required on heirloom?)