For example, in AD&D, a 9th level cleric can build a place of worship. In other words, collecting 110,001 xp lets you make a building.
Turning this on its head, you can try to make a place of worship whenever you like, and if you pull it off it'll earn you 30,000xp.
What happens to the game if you reverse all of the perks this way?
Here's all of reverse-perks I worked out:
The PerksWhen a cleric builds a place of worship - a building of not less than 2,000 square feet in floor area with an altar, shrine, chapel, etc. - they earn 30,000xp.
When a cleric attracts 100 followers, they earn 30,000xp.
When a cleric constructs a religious stronghold (a fortified castle, monastery or abbey containing a large temple, cathedral, or church of not less than 2500 square feet on the ground floor), they earn 100,000xp.
When a cleric clears an area and establishes monthly taxation of not less than 9sp per inhabitant of the area, they earn 50,000 xp.
When a fighter establishes a freehold, some type of castle and clearing the area in a radius of 20 to 50 miles around of hostile creatures, they earn 30,000xp.
When a fighter attracts and pays a body of men-at-arms, led by an above-average fighter, to the well maintained freehold, the fighter earns 30,000xp.
When a fighter establishes and collects a monthly tax of 7sp per inhabitant of their freehold, they earn 10,000xp.
When a paladin finds and retains a warhorse of unparalleled quality, they earn 6,000xp.
When a ranger attracts a body of 12 or more followers, they earn 50,000xp.
When a magic-user finds and learns a new spell, they earn the spell's level squared x1,500xp (e.g. finding and learning a 2nd level spell earns 6,000xp, a 5th level spell earns 37,500xp).
Mostly I'm just playing around. One of the things I've said in the past is that class/level perks insulate the character advancement from the events of the game.
Stuck fighting undead baby kraken at sea? No worries, you'll still earn that precious xp that will make you a better magic-user or sneak thief, even if that seems a little weird.
In a way, it's a pre-packaged heroic character arc on rails, the player-side mirror image of a linear adventure.
Another observation: rules like this prevent something from becoming the focus of the game.
For instance, if you automatically attract religious followers when (as a cleric) you acquire a certain amount of treasure (xp), the GM and players don't need to get into the business of trying to attract religious followers. It just happens, as if by montage.
Similarly, in The Regiment (Apocalypse World-based WWII game), there's a downtime move that causes hours, days or even weeks of leave to breeze by in a single die roll, because while downtime is vital to war-weary soldiers, the game isn't about the downtime.
Maybe that's what you want, maybe it's not.
One of the things I'm playing around with these days is mechanics that have the greatest possible effect on the buildup of campaign capital, and I think this sort of design decision is a central one.