I've been working on a progression system for Heroes to level-up which you can view here https://maister-mule.itch.io/the-book-of-fates
We've playtested 'The Book of Fates' for a year in a number of different groups but there are so many variables it's become really difficult to gauge the balance. Is it fun? Does it add drama? I'd love it if people could give it a read and maybe play it in a few quests. Feedback, comments and ideas for outcomes all welcome.
I hope to make further refinements and updated versions but any thoughts are welcome. 
Some of the design ideas:
Keep it simple in line with the game.
No additional cards or dice.
Focus on affecting the stats of a player and avoid adding more skills, spells or components.
Add some jeopardy and real consequence.
Prevent a group of Heroes becoming stale.
