The Factorio Devs made this Blogpost some time ago about how to go forward with development. It basically boiled down to developing a big (paid) expansion pack. I would be totally fine with Stardew Valley doing the same. However, going for free updates was discarded for Factorio because they have a “smaller but more dedicated audience [compared to Terraria and Minecraft]”. I don’t think that applies to Stardew Valley so free updates might be the way to go.
They did some great fan service before. In Borderlands 2, there is the NPC Michael Mamaril in Sanctuary who can randomly give out guns when talked to. That NPC is a tribute to the real Michael Mamaril, a fan that passed before Borderlands 2 was released