3. I might have to come up with rules for mystical tattoos. Should they work like rune paper?
How much power do you want to give the players, and how much do you want it to cost?
If there are no limiting factors, characters can be walking runebooks, with power far beyond what is typical for their profession and level. A couple of obvious limits you could put on them are:
1. Limited number of runes per person.
2. No magic on the body part that is tattooed (no magic rings if arm/hand is tattooed, etc.). Failure to follow this will result in random power interaction (think potion miscibility table).
3. You have power embedded in your skin. IT HURTS. You are constantly 1 hit per tattoo below your maximum hits, and each one causes 1 hit/PP embedded in the tattoo upon casting.
There are any number of ways you can work it, those are just examples. Decide what you want the balance of power to be in your game, and create a logic structure that enforces that balance. It's your game, make it your own.