I use Milestone Advancement largely because I'm lazy. I could put in the work for an XP system, but it's not of my interest.
We use 5e D&D for our home campaign. For levels 1-8 the party levels up frequently and at a steady rate, provided they are doing something interesting (no leveling up for staying at home). Beyond that the party needs to impress me with their accomplishments to justify a level up. Here are some categorical examples:
Discovering a Lost Magic
Uncovering a method of Invisibility, non-transformative Flight, Resurrection, or Teleportation is an automatic level up for the party. These magics are hidden throughout the world, usually somewhere remote and dangerous.
Changing the World Unalterably
The whole point of the campaign was to more or less give the players the tools and opportunities to smash the system in ways they saw fit. Any purposeful acts with monumental social or political shifts will grant a level up.
Getting Rich/Finding Treasure
Finding a significant (upwards of 10000gp in value) treasure from somewhere dangerous will merit a level up. Acquiring an Artifact of significant power has the same effect.
The general rule is: if it's in doubt that what you did it significant, then it wasn't significant enough. Also, if it's a partial success or spectacular fail, if it did so interestingly it still gets a pass.
Some Impressive Things:
- Pulling off a coup
- Capturing a legendary monster
- Discovering, recording, and mapping a place that has been undiscovered since the apocalypse.
- Gaining additional Senate seats in Twilight City
- Putting a player on the Archmage council of Chronulus
- Going back in time and significantly changing history
- Leading an army to victory
- Steal the powers of the Dread Sorcerers of the Seventh Door (maybe 4 out of 12 or so?)
- Corral the powers of some Saints (again, 4 or so)
- Steal the power of the Sorcerer King
- Blackmail a country's leader
- Kill someone immortal
- Found a city or country
- Pacifying a known wilderness region (explore, map, conquer)
- Publish a famous scientific article in a Chronulean journal
- Publish a famous book
- Stop/Unleash (intentionally) a deadly plague
- Legendary Martyrdom
- Become a Saint
- Pull off an elaborate heist
- Start/End/Prevent a major war
- Form an Army
- Marry someone very important (relations aren't enough, you gotta join bank accounts!)
- Patent some discovered spells (2-3 probably, patenting invented spells only gets you gold)
- Find a complex of The Oppression Engine
- Find a route to the Silver Palace
- Bring back a lost language from Leviathan
- Found and cement the position of a criminal gang
- Bust the dragon Volectra out of her comfort-prison by putting on the World's Best Play
- Assassinate or otherwise forcefully retire an integral political figure (An Archmage, a Fury, the Solar Emperor, one of the 3 chief Drow Matriarchs, the Diamond Orc, the Dragon-King of Kobara.)
- Learn how to regularly do Astral Projection (Thank you Emmy Allen for this one!)