What if there was pieces of equipment who's full potential can only be unlocked through energy? This doesn't need to be debilitating for free players either.
Say the max you can reach is level 5 on these special pieces of gear. To reach beyond that threshold, you need to imbue it with energy and thus allowing you to further level it to it's max level.
I'd recommend this being for weapons that have multi-stage charge up moves or armor and weapons with a higher than normal stat growth.
I think, the current state of the game doesn't require gear THAT powerful.
That and since you're already paying to find items and craft items, paying to upgrade them might be considered a little outlandish.
I won't argue if they do end up doing this, but there are better methods they could employ that don't require directly paying to upgrade the item's potential.