Microblog: Diegesis Parasites
In the vein of The Silurian Game, here's another, slightly more gamified way of encoding the nature of the roleplaying table diegetically.
In the Parasite Game, the players play not as Grognar the Barbarian or Xofrangilë the Elf Mage, but as hyperdimensional parasites controlling them. These dubiously mutualistic symbionts feed on the concept of value, typically stored in the form of gold and gems, and grant increased vitality and power to their hosts. In exchange, they can gain complete control over their hosts at any time (i.e. while the players are playing their PCs). Each "level" is the amount of sustenance necessary for a parasite to create an immature clone of itself which can infect another living creature if allowed to slither down its throat. These parasites are part of the same hive, sharing information and goals amongst themselves. In game terms, when a PC reaches level 2, they can recruit a level 1 retainer controlled by the same player. Because this recruitment is diegetic rather than purely mechanical, however, any living creature can be infected and added to the hive: devious players may attempt to infect powerful or influential NPCs rather than simply recruiting wayward adventurers.
This is a very different way of encoding the medium of roleplay diegetically, but I think it's kind of a fun idea that incidentally puts a lot of focus on the domain game, since you're effectively acting out Invasion of the Body Snatchers from the perspective of the body snatchers.