From the thread about shaft mines and their unhappiness, I got this idea. If it is feasible and acceptable, I want to have an improvement to upgrade when appropriate tech is present. Otherwise, it still is the at the latest build. Just an example, what about cottage turning into town only when Coinage is researched?
Actually, I want to see if it is possible to merge trade posts with cottage improvements so they can upgrade together but with tech requirements. The same can be said for Workshop upgrading to Industry(the earliest one, forgot the name) only at whatever the tech was for that industry.
Reason:
That would make shaft mines causing unhappiness a more reasonable if we have shaft mines only available at Steam Power.
Of course, if it is either or both not interesting or not feasible, forget it

.