Just in broad strokes, how would you like the rune magic to be done?
Usage of runes in warfare is enchanting weapons, armor, jewelry, or machinery with a huge variety of effects, or used in doom anvils which function like an artillery piece.
Personally i would like an emphasis on offensive lightning magic with pronounced visual effects and powerful sounds.
Doom anvils might be out of his league lore-wise, but if not - i would like them to be a ranged weapon, with 2 or 3 anvils to choose from with different effects. They don’t produce overheat, they have ammo that recharges overtime.
Ultimate ability - when you press it, it activates the lightning rune on your melee weapon, your next strike will have a bonus magical effect. Upon contact, the rune will do a chain lighting staggering multiple foes for several seconds, dealing moderate damage… each enemy hit with the weapon strike will become a node to pass electricity. Have it a short cooldown of 30s.
One 30 lvl talent would change the rune effect with overhead attacks - it would make a bigger damage with lightning strike from the sky, at the cost of doubling the cooldown timer.
His armor or shields could be enchanted so that when he gets hit, an attacker gets a zap of shock, staggering and dealing damage.
Runesmiths also play a role of magic defense, dissipating enemy spells, so there could be a talent that will sap the energy out of vortex storms and leech grabs, nullifying them and recharging Bardins ultimate.