As far as I understand any weapon or armor of any rarity will be eligible for enhancements. This means that even strongest items can get extra +stat, +damage, +something else. This can make some already powerful items way too powerful.

Also it just breaks the idea of crafting your own "normal" items and improving them. Instead you'd better of buying some epic and improving it.

It will put limit on how much a certain item can be improved. Divine items for example will have like 0 improvement capacity, while properly crafted white ones will have a much bigger potential. Also you can add different tiers of recipes making sure that a 2h sword crafted with much more rare materials (for example using some diamonds and runes) will have same stats as normal 2h sword but a MUCH BIGGER capacity "improvement capacity"

Example of implementation:
2h sword made of cheapest materials has improvement capacity of 6
2h sword made of mediocre materials has improvement capacity of 10
2h sword made of superior materials has improvement capacity of 20

+1 stat enhancement takes 4 capacity, +elemental damage takes 3 capacity, +random proc (like 20% chance to set wet) takes 3 capacity, +skill takes 5, +damage boost (whetstone) takes 2 and so on and so forth

Also do not restrict application of same boost several times but simply make it more expensive with each extra time it is applied. +1 stat takes 4 capacity, +2 stat takes 10, +3 takes 18

All numbers are just examples and are surely not balanced and thought-through