Runes are cool for three reasons 1) they are a unique alphabet/system of writing something that has fascinated me since childhood when I memorized the Phonecian alphabet from an old encyclopedia 2) They involve rune poems which give a mysterious sense that "Fehu," for example is much more than just an "f" sound, and 3) these rune poems usually involve kennings making them thrice cool.
Just to give you an example of what I'm mean:
Wealth is a source of discord amongst kin;
and fire of the arms.
Ulcer is fatal to children;
death makes a corpse pale.
So, what I want to do is incorporate rune magic into my campaign. I don't see the class as much different from a cleric, it's the mechanics of the magic that will differ. To me that's what's interesting that there are different kinds of magic in a game world. So, I would focus on the things that make runes cool and different.
First, they are not spells to be cast, runes have to be inscribed. So, they are either ritual like inscriptions that affect an area or thing inscribed, or they must be inscribed on amulets ahead of their time of usefulness.
Second, The magic should be cool in a combinatory way. That's sort of the point. There aren't 24 rune spells (because of the 24 Eldar Futhark runes) that would be very boring. There are spells that are brought about when you combine Fehu + Kaunan, or even Kaunan + Fehu.
It sounds like the first requirement might make a runecaster pretty useless on an adventure, but I'm not so sure. Those spells that later editions of D&D call "utility," like something you pull out of a mop closet, are the things that would be the most useful in real situations. I'm thinking something like Othila (Estate/Property) + Kaunan (Ulcer), for example, would make a door brittle and allow chracters to smash through locked doors or even chests.
Two ways of parcelling out this power to the runcaster that I'm thinking of 1) the dreaded spell components, i.e. to inscribe take some expensive dyes or gold leaf if you aren't actually carving into stone. To draw on human flesh would take expensive, aromatic oils. 2) Knowledge of the runes is incomplete. You could accomplish this by saying you can't achieve this rune combination until level 3, but that seems very video gamey to me. I'd rather say, you don't know what the Kaunan rune looks like. Hmm, I'm sort of unconvincing myself. Maybe the chracter knows all 24 runes to start, but needs to know the rune poems to use them effectively. That would be something they could be searching for in play, the way old school mages hunt scrolls.