I can't think of a fancy name right now. But here's how it works. Aside from the exp units get in battle. A percent of the exp goes to a global pool after battle. Say after battle all your units get 100 exp each, a certain percent of 100 exp goes to the global pool. This percent is small, 1% after first tech to unlock it, up to 5% with additional research. Units, champions, and sovereigns must be inside a city for them to get bonus exp. If they get bonus exp they will lvl up like normal.

Some new techs

Warfare tech that "... allows your troops to share their battle experiences" or something similiar

Adventure tech that "... allows your adventurers to tell their tales to others" or something similiar.

Maybe implement a cap on the bonus exp that units, champions and sovereigns are able to receive.