Here's an idea. Rather than increase the potential on Invulnerability, and at the same time reduce the usability of the ability, put an absolute minimum level on the ability. This would guarantee that there won't be anyone running around at level 1 with an invulnerability item on. I don't know if cursed shards can be created or not but I have seen many cursed items that reduce the minimum level on an item in trade for say -1 Strength. If minimum levels on abilities are implemented seems like it will be a lot easier to make some of the abilities that currently are not available for crafting available without unbalancing the game. As it is, modifying the potential on abilities just limits the usefulness of the abilities and begs for further nerfs in the future.