First time trying to come up with a suggestion for a skill just to see where this goes.
Currently there're only 2 skills that are based off of runes and it's runecrafting and magic. I feel that runes, being a huge part of the runescape universe, have little use outside those 2 skills and it's kind of a conundrum of sorts.
My first skill concept would be golemancy:
Generally speaking, this skill would pretty much be a sort of create your own "combat pet", where it requires the player to interact with various pre-existing skills in order to first make and then improve it. You can still have your regular pet out without worrying.
For example: you'd be able to make a golem with an iron torso, wooden hands and legs and head out of fire, water, earth or air based on the rune type.
The options would be wide and each type of golem would have their own set of abilities, perks and disadvantages, like having low attack speed with iron/rune hands, or have higher hit points with a dragon core, while casting weakened versions of the standard spellbook. The golem's HP is based on the level of the items used to craft them and scales with the player's own HP level. Players can also have the golems wield their own weapons, but are limited to daggers, swords and crossbows, requiring higher golemancy level, as well as the required level a player would also need to have to wear better weapons like an ACB or osmumten's fang; the golem's attack speed would be limited to the weapon's attack speed.
Golems would also have slots to store runes to cast combat spells if ordered to, and bolts to range down opponents; Enchanted bolts have their effects reduced by 66%. These styles require the player to interact with the golem while being out of combat.
The skill would be leveled either through combat or any activity in the game with a certain multiplier of the overall xp going towards golemancy, or by crafting multiple golems, having their xp based off of the level of materials used.