Remove the utility that Garrisons had (if they do implement a “farm” aspect, make it so those only effect cosmetic items, not something that would increase your player performance).
Future proof it: One of the things that made Garrisons a failure is that they abandoned it once Legion was released. Release new furniture items (or furniture crafting recipes) in subsequent patches. Make sure that the housing system doesn’t break in future expansions, make it so that it receives needed updates in future expansions. DON’T ABANDON IT ONCE THE EXPANSION THAT INTRODUCED IT BECOMES LEGACY CONTENT!