|Eternally Lost Boy
Perhaps one dealing with handling how a rampaging spawn of Rovagug can impact a region. There's more of them then just the Tarrasque so there is a selection to choose from and the others I believe would fall in a CR range that PF2E should be able to handle as either as the final encounter in a campaign or finding some alternate means of halting their rampage as the finale of the AP. I think it would exciting to see the PCs at level 1 having to take part in an evacuation as a spawn hits goes through their hometown and as they get stronger have to deal with the effects that one of these mythic beings have on the region they are currently touring and then getting to the point where they can more directly take on these forces of nature.
Even with the 3 module AP model having a party spend half of their career trying to make sure the one in a coma in Osirion remains asleep I think would be rather entertaining.