"Tall" Legacy Trait: Liberated Cities become Client States

"Together We Rule" introduced the Sumerians and Singaporeans, whose Legacy Traits are explicitly targeted at allowing you to become a "tall" (an empire with fewer, better cities) empire instead of a "wide" one (an empire with more cities).  They go a bit of the way there, but using either of their powers to attach/merge is still expensive and inefficient.  There's a third mechanic to convert your empire into a tall one, and that's to turn your cities into Independent Peoples with the "Liberate" action, but the resulting city has only a small connection to you, not representing its historical ties to your empire.

Legacy Trait: "Liberated Cities immediately become Client States with Patronage of X"

This could represent the Hanseatic League or any number of medieval trade unions across the world, loose confederations, or alliances of city states.

This allows your empire to immediately shed cities down to whatever level you'd like while still retaining some money and science from them, and ideally being able to protect them, too (attacking a client state should require declaring war on its patron).