I added in a line of civilian promotions in my mod-in-progress a while back. Basically Libraries/Universities/Research labs act like Barracks for Workers - newly trained units start with some XP. I haven't figured out how to add earned XP to worker actions yet so the only way to get an experienced worker is to replace them when you get the buildings.
I didn't improve build speeds with Promotions. Basically the promotions give increased sight rage or increased movement based on terrain - Woodsman, Altitude Training - and a few gated by technology (Defensive Embarkation and Paradrop, for example) mainly to allow workers stationed in a particular city to be trained specifically for that city or settlers to be trained for reaching a destination quicker/safer.
There's two major issues with it - the AI doesn't replace workers as frequently as a human player might (they almost never disband them after they're built) nor do they station them at a particular city where these promotions might benefit most.
And secondly, all XP is lost on capture, if a barbarian or opponent captures a worker, even if you capture it back it will revert to level 1 with 0 XP.
I didn't improve build speeds with Promotions. Basically the promotions give increased sight rage or increased movement based on terrain - Woodsman, Altitude Training - and a few gated by technology (Defensive Embarkation and Paradrop, for example) mainly to allow workers stationed in a particular city to be trained specifically for that city or settlers to be trained for reaching a destination quicker/safer.
There's two major issues with it - the AI doesn't replace workers as frequently as a human player might (they almost never disband them after they're built) nor do they station them at a particular city where these promotions might benefit most.
And secondly, all XP is lost on capture, if a barbarian or opponent captures a worker, even if you capture it back it will revert to level 1 with 0 XP.