I love me some 13th Age. It is a well built game and highly enjoyable, one you should check out if you are not familiar. I’d describe it as D&D 4th Edition without all the compromises that held 4e back from being a truly awesome game. Having said that, I know plenty of groups that will only play D&D 5e or Pathfinder.
For GMs in such a group, I recommend doing a bit of stealing from 13A. There’s plenty of goodness in this game that is relatively easy to add into other games. Here are a few ideas.
If there is a single mechanic that is truly iconic to 13th Age that is highly stealable, it is the idea of Icons. Every setting has more powerful persons that may take an interest in the player characters. These powerful persons have allies all over the place, and they will help you where you can.
2) Ignore XP and Level Where Appreciate
Old school players will remember having to find a teacher to train you and having to pay them, which ate the gold you found. So even if you got enough XP to level up, you didn’t level up until you got back to town and trained. By today’s standards by players use to modern versions D&D or Pathfinder, that can feel a little punishing. So instead, steal from 13th Age and just ignore the XP gained and level up at an appropriate place in the story or between arcs. There are many good justifications for this: leveling up is too fast, less mechanics for the GM to keep track of, or you feel it makes more sense in the story are just a few.
3) One Unique Thing and the Skills
Player characters are unique, even among adventurers. They should have an idea about them to help crystalize the idea of their character and emphasis their uniqueness. Steal the one unique thing and help them have their character’s personality shine through.
And if you want to take this one a step further, steal the skill system. If you can justify using your skill of bring a “Noble” to add your proficiency on Charisma checks when trying to influence people, the “Rebellious Youth” skill to explain how you add your proficiency to Strength or Dexterity checks that would normally use Acrobatics or Athletics when climbing a rope or walking across a narrow rooftop, and the “Royal Education” skill to why you are better with Intelligence checks when trying to unravel historical, religious, or arcane mysteries you encounter.