Magic Words

Imagine a campaign where the first PCs are the ancients, who create the spells the rest of the PCs will later have to uncover.

If you need a structure to help come up with spells, Carcosa has a good one: weird monster, spell to summon it, spell to bind it, spell to banish it, spell to make it do your bidding. Having monsters running around that are primarily interacted with using spells like these is a good way to prompt spell creation.

Ask your players what the magic words are, and record what they say (with permission). Make them a bit too long for anyone to remember. After the session, write them down.

Before too long, have a generations-long timeskip. Build your world to plan for this, either with a smaller area for the 'ancients' section, or a short 'past' entry and a short 'future' entry for each location.

Thenceforth, to cast spells, players have to find their words written down somewhere, and repeat them exactly or risk interesting magical mishaps. If someone manages to remember one perfectly from earlier, let the feat of ancestral memory stand.

Brief Example

Session 1: Outer beings have broken into the world, and you, the ancients, must handle them how you see fit with your godlike command of magic.

Session 2: Centuries later, a rediscovered ritual unleashes one or more outer beings, and you have to delve into history to find your other rituals.