So I am working on a modern horror / fantasy and an idea i have had is to use tattoos to permanently ink people with magical glyphs as an extension of old world rune magic.
Some of the ideas I have had are:
- Stylised shields or symbols of defence give the inked the ability to cast Barrier.
- Images of appropriate animals can be used for enhance i.e. bull for strength, bird for agility
Various effects for utility magic
- Key on the hand gives a boost dice to Skullduggery checks to open locks
- A nchor gives a boost dice to checks made in the water etc.

What are peoples thoughts, what other tattoos would you include?

I think that most of said runes would function best as reflavored Item Attachments and/or Cybernetics. So first I'd raid those rules for their mechanics and design principles.

My second suggestion is to avoid minor circumstancial bonuses to checks. The benefits rarely outweigh the cost of having yet more details to track. Instead, many rune-bearing items could simply be "the right tool for the job".

For Example:

Lock & Key Glyphs: These glyphs magically lock the portal they're engraved unto, such that only the bearer of an item with the matching glyph may open or pass through it. A successful skulduggery check might indicate the character was able to mar the lock glyph, or fake a key glyph. A magically-glyphed physical lock might even require a specific key bearing a specific glyph to open (or a set of properly glyphed lock picking tools to even make the check).

Most Glyphed Armor & Weapons can simply have better profiles than their mundane counterparts. I'd grant bonus Damage/Soak, but be leary of making weapons more accurate; because accuracy and evasion can't scale like damage and armor/soak can.

Most abstractly, an Anchor glyph might simply fix the relative positions of two objects, and the applications of such an enchantment are endless. Bonding dissimilar materials, levitating structures anchored in mid-air above their foundations, nearly-frictionless machinery (with moving parts that never actually touch).

Sounds awesome and I agree with Cantriped's extension beyond just tattoos on people.

My home brew magical system includes Runes as one of the three types of spellcasting. Magical symbols of power are needed to cast spells either by speaking them (Word-casting skill), thinking them (Thought-casting skill, high tier unlock that has advantages since its the way the "gods" do it), or writing them (Rune-casting skill).

Rune-casting is used when marking runes upon people (tattoos), places, or things. While this makes them the slowest to cast, it makes Rune-casting unique since you can mark spells upon a person, a weapon, a shield, a doorway, a chest, etc. Thus it's used for buffing people, buffing items, setting traps, and such.

I would suggest to use them as magic implements. You could re skin the runes from terrinoth as tattoos or make up your own. If there is a magic skill in your setting you could use a tatoo as a magic implement maybe limit it to a certain magic spell pr tattoo.

If you dont have the Terrinoth book Runes are a magic implement that has a passive ability that can be activated by anyone but when used by a magic user with the appropriate skill it functions as a implement.

Example re skin as tattoo could be the lighting rune.

Activation: Until the end of your character's turn, the lightning
strike rune counts as a Ranged weapon with the following
profile: (Discipline; Damage 8; Critical 3; Range [Long];
Auto-fire, Disorient 3).
Implement: When your character casts an Attack spell, the
first Range effect they add does not increase the check’s difficulty.
In addition, they must add the Lightning effect with
no increase in difficulty. Attack spells cast by your character
increase their base damage by five.

If you dont have a magic skill in your setting maybe they just have the activation ability.