"Spells, I know as many spells as I know runes, but I am limited to cast them, and be really, really sure when to cast a spell or not. My space is limited."
Ink rune magic is rune based magic, where you tattoo runes to your very own skin to wake the power of the rune and direct its power from your body and mind.
Rune is the formula and knowledge.
Your skin is the arbiter.
Your mind is the discharger.
If your character practices Ink rune magician, he learns runes as others learn magic in setting. His magic is more ritual type, than casting. He can tattoo the runes in advance, and cast them later. When tattooed rune is cast, it wears out leaving no power, but only a tattoo behind.
Basically (in Dungeons & Dragons) instead of resting to gain spells, Ink rune magician prepares spells by tattooing them. The spells he can tattoo per day might be the same as wizard can cast and memorize a day. There is a flaw though, every spell Ink rune magician prepares and casts, leaves a permanent mark on his body and takes space. His skin is not endless canvas, there will be a day there is no space for new runes to be tattooed clearly enough.
Other problem is that all cast and prepared runes are visible. At some point, his skin will be so tattooed with runes there are no hidden places to use, but he must start to tattoo more visible places. If Ink rune magic or tattoos are a taboo, that can be a real social problem later.
Another problem is, the more you cast the less you can cast later as every rune takes space from your skin.
Rules Ideas For D&D (3.X)
Ink rune magic is a skill you can learn. Basically it is available for every character class as a multiclass when character has possibility to learn it. It cannot be only class. Ink rune magic (class) uses same statistics as wizard.
Tattooing a rune is Intelligence + Ink rune magic skill difficulty of 5. After every rune over character's Constitution the difficulty to successfully tattoo a working enchanted rune is +1. After every 5 tattooed runes above character's Constitution gives -1 negative to roll Charisma based rolls.
Casting successfully tattooed runes is +5 easier than casting normal spells as the body and skin of the caster is already enchanted with the rune, and the rune is successfully charged with the power to be released.
Even though there are major flaws in rune magic, they are easier to cast for multiclass characters who aren't into wizardry from the beginning of the character creation.