Liches are pretty well known. I can imagine some wizardry studies thousands or tens of thousands of years ago working out the rituals and magic to become a lich.

So I thought, why aren't there any wizards working on and discovering a process to become an even higher being? Such as a powerful elemental? And then I thought what about an intermediary phase, in this case for fire elemental, that provides benefits to the Wizard like a lich, but works differently.

Eternal Flame - Acts like a phylactery, after all the required rituals, but works differently. The eternal flame must consume fuel, it cannot be a magical flame, and it probably should use wood, because burning wood is just so much more awesome than a dais of oil. As long as the fire burns, the wizard will live and never age. If the wizard is destroyed they will reconstitute from the flames (like a phoenix?). If the flame is extinguished then the Wizard can be killed (but is not destroyed) and can age. The Wizard will have to perform the rituals and material components again to achieve Eternal Flame.

I imagine a powerful wizard has his Eternal Flame hidden deep in a catacombs beneath a family's ancestral home. The catacombs is guarded by Golems who also operate the Eternal Flame (providing fuel to the fire, removing ash, etc.). And has a host of servants, maybe Kobolds(?), who maintain the catacombs and bring a steady supply of wood for the fire.

Now the progression to an elemental, I haven't worked that out mechanistically as if for role play. But I'd imagine it can be any other means that a player character attempts to achieve godhood, or in the case of the more powerful elementals, beyond godhood.