I use a series of Runes representing concepts or elements in the world that have one ability per tier of the PC, so it scales with you. The key thing is that the abilities or spells per tier need to be from a variety of classes or at least not the player's class. For example: Protection
Tier 1 Grants a refreshing shield of Temporary HP that gains 5 HP when you kill an enemy of CR ⅛ or greater. 10 max. Max increases by five per tier.
Tier 2 Magic Circle 1/day
Tier 3 Wall of Force 1/day
Tier 4 Forcecage 1/day
Tier 5 Invulnerability 1/day (no material components)
This is also a good tie in for new adventures because enemies could be looking for other runes or the players could want to find more.