The 1.5.5 update for Stardew Valley has just gone live, and in the patch notes is a suggestion that the 1.5.6 update might contain some new content. "There may be some new content in 1.5.6, too, but it's not going to be anything huge like 1.5." Previously, Stardew developer ConcernedApe had said he wasn't sure if he'd be releasing new content for the game as he was busy working on his next release, ConcernedApe's Haunted Chocolatier.
Modding also seems to be a big part of Stardew's future, as the 1.5.5 patch and the announced plans for 1.5.6 both make modding easier. Along with "various internal changes for mod authors," 1.5.5 moves the game to the MonoGame architecture, futureproofing it and allowing mods to access more than 4GB of RAM. Meanwhile, ConcernedApe is currently working with PathosChild, creator of a popular Stardew Valley modding tool, to add more mod support with update 1.5.6, including some "holy grail" changes. PathosChild has even been given access to the source code to help with the task. So, regardless of if update 1.5.6 has new content or not, PC players will likely be seeing a bevy of new content for Stardew in the form of future mod releases.