I'd love to see Districts have useful lifetimes, deteriorate over time, and eventually become obsolete. You could, for example, build The House of Wisdom, or The Library of Constantinople, or the Jixia Academy, and be a center of academic learning in the Ancient or Classical World (links go to Wikipedia pages). But those schools shouldn't still be leading-edge hundreds or thousands of years later. They may cease to exist at all. Their legacies may last long after they're gone. After all, we still use Arabic numbers and Arabic astronomy today. In Civ terms, their Science output might transform into Culture over time. From a gameplay perspective, this would require the player to give thought to when to build a particular District. You might put the pedal to the metal in the Medieval period, then find yourself slipping during the Renaissance and struggling to hold on in the Industrial Era (read: Ottoman Empire, Shogunate Japan - one of them turned it around, the other did not). You could surge ahead in the Ancient and Classical periods, then lay dormant for a while, then burst across the finish line in the Modern & Information Eras (read: China). Venice and Portugal were once economic powerhouses. What happened to their Harbors and Commercial Hubs? In Civ VI, they'd still be there, just as profitable as ever. There should probably be some way for a District to be revitalized and/or maintained - Oxford University and the Sorbonne are still around - but it should be unusual. Expensive, perhaps, or with difficult preconditions, but worth doing once in a while. Perhaps, as with Arabic mathematics, Districts transform over time, contributing different bonuses than when they were brand-new.