Or just turn the part loose, as the saying goes, no plan survives first contact with the enemy, and player are the enemy of dm plans.
When making a world for campains i long abandoned making a large scale plot and just made citys and various areas that are as reactive and adaptable as possible, then just wait on players to kick the hornets nest and just let them keep escalating the problem till somthing gets labeled as a final boss and i just quickly wip up a sheet for it.
Like once one player desided he wanted to pee on city walls to mark territory, when guards came to fine him the party killed them, rather than explain this was wrong to the party i just had the guards be servents of a tyrannical noble, so the players had to then move stealthily to avoid being swarmed while finding allies who who still sell to them, while trying to overthrow the noble.

Lesson here is you don’t need to plan escalation, the players will take care of that.