The answer, which is rarely this clear cut... is to have them be sold and the new owner move them back to SD and work with the city to make a stadium that's modest, in the spirit of the laid back SD style and shares with one of the local unis.
SD fans were passionate and while I think they'd be embraced in St. Louis, among other cities, especially if there were no stadium drama... the Chargers need to go back to SD under new ownership.