A tattoo that's blank and does nothing, but you can equip it with items and select certain spells. Then merge the items/skills/spells into the tattoo and essentially it becomes a familiar which uses the infused items and has its bonuses.
It has to level accordingly to make the items stronger, works like the Halidon or Jigorokano familiar, and the EXP is distributed appropriately to each item/stat.
Obviously this will have to be a costly process, and it shouldn't level at the normal tattoo rate.
Thoughts?