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.
It's not new information that any and all singleplayer DLCs that Rockstar planned to release for GTA V ended up being scrapped and reworked as GTA Online expansions. What is new, however, is the bit that the latest Diamond Casino & Resort content pack had its start as a dedicated singleplayer DLC.
Popular dataminer Tez2 said that "what Rockstar was planning to do with the single-player DLC can be compared to adding a whole new map expansion." Apparently, they intended to redesign large portions of the game world to host Alien and Zombie-focused DLCs.
Rockstar intended to feature a major Casino heist expansion, which is likely where the latest GTA Online DLC was going to tie into the rest of the singleplayer GTA V experience. Sadly, this won't happen anymore, but at least the content wasn't scrapped altogether.