Mentors and Mastery
In my games, we use an XP system called Mastery. The party gains 1 Mastery for each session played, as well as whenever you defeat a Named Villain. I’ve recently thought about adding campaign-based and character specific Mastery triggers, but haven't tried it yet. You can spend 5 Mastery to add 1 to a Stat. 10 Mastery gets you a Skill or an extra 5 HP.
If you form an alliance with them, The Old Guard will act as mentors, teaching you forgotten methods of warfare. These skills only cost 5 Mastery to learn.
Feet of the First Men (Taught By Meshigam): Through a brutal training regimen, Meshigam taught you how to stand with defiance. You cannot be knocked prone. You may gain a D4 Mighty Deed instead of moving on your turn.
Blood of the Serpent (Taught By Taro): Taro shared a small portion of his curse with you. Whenever you take 5+ damage from a single melee attack, inflict 1d8 damage to the attacker.
Withering Glare (Taught By Mer Meridath): Mer Meridath taught you a spell, one of the few that mortals can withstand. As an action, you may glare at an enemy. Until the round ends, they subtract 1d4 from each of their rolls.