GTA V had 3 single player DLCs planned
From a recent article from Eurogamer we learnt that GTA V had 3 DLCs planned, code named: GTA, Trevor and Dead (this piece of DLC seemed to hint at an expansion similar to RDR's Undead Nightmare). Leaker Tez2 had to say this about the zombies DLC:
"What Rockstar was planning to do with the single-player DLC can be compared to adding a whole new map expansion," said Tez. "They had plans to redesign 80 or 90 per cent of the map to fit a specific theme - one of the episodes was supposed to feature a zombie apocalypse. Rockstar would make changes to the whole map to fit the zombie apocalypse theme."
You can also check the leak showcasing the SPZOMBIES in the menu:
Unfortunately it didn't come to fruition as Rockstar have abandoned the DLCs in favour of GTA Online.
The Trevor DLC would've been "based on Trevor's story after the ending to Grand Theft Auto 5, in which he would work with the IAA." Tez posted about this DLC all the way back in 2015 on a post on GTAForums (The main characters will rob the casino) that hinted at an Oceans 11-style setup and mission. The idea of working with the IAA was repurposed in an Online update, The Doomsday Heist:
"it turns out that most or some of the online updates that we received in 2017 and 2018 were based on the planned single-player DLC." - Tez
"While there's no casino, you work with the aforementioned IAA as part of the narrative, and it utilizes the malevolent 'Clifford' as its neural network antagonist - a name which cropped up during the 2015 casino leak."
Also the newly opened The Diamond Casino & Resort also had its roots in a singleplayer DLC. According to ex-leaker Yan2295:
"The casino was supposed to come to GTA5 as single-player DLC as well as to GTA Online, but as many already know, the single-player DLCs got cancelled thanks to the success of GTA Online."
From the looks of it most of the ideas Rockstar had for SP DLC were later reused and formed to basis for some of the MP updates that players got in GTA Online.
The Casino updated was pushed back so long because Rockstar had to think of a good way of implementing gambling in Online without " falling foul of gambling legislation and getting the lawyers involved". It seems they found the perfect way of implementing microtransactions in RD Online and they used the same concept in GTA Online:
"The casino update was canned since, because of Shark Cards, laws in some places would see that as online gambling, which is illegal in some of those places, especially considering many players are teens or kids," Yan explains. "But recently, they found a way to make it work, as we all saw gambling being added to Red Dead Online."
Tez echoed a similar sentiment: "In Red Dead Online's case, you use cash to gamble and you can purchase gold bars from microtransactions. The extra currency ensures there's a layer of safety for Rockstar from legal trouble, even though that doesn't apply to all regions or countries as some may have trouble with the existence of gambling itself - and not the way to acquire cash."
In the mean time, GTA V became one the most profitable games in history making over $6 billion in revenue. You can read more about GTA V success here.