From GameWatcher: “GTA V is no slouch when it comes to content, but the new Diamond Casino & Resort DLC wasn’t supposed to come out Online-only, but rather as a singleplayer story expansion, too.
This information comes via recent datamines, which point to substantial singleplayer expansions for GTA V, much like Ballad of Gay Tony was in the case of GTA IV. Of course, this never came to fruition, but it is interesting to think about.”
Source: N4G PC GTA V Casino DLC was supposed to feature Singleplayer