A group of modders is attempting to recreate the original Red Dead Redemption inside of Grand Theft Auto V. The mod is planned for single-player and the team is currently working out ways to bring over content from the original game in a timely fashion. The first beta for the mod is planned for this Summer.
"Red Dead Redemption V is a modification for Grand Theft Auto V. Our goal is to bring you the full map with additional content which will be unveiled later on," says the team in the mod's FAQ. "The framework is all there, we are constantly finetuning the methods we use to convert the game. .White aims to get as close to the original as possible. Out of all the other Rage games released before GTA5, RDR is the toughest one to crack, so please be patient."
The mod's current progress is the result of two years of work, with the current download size coming in at 2 GB. The mod team is porting from the Xbox 360 version of the game.
It's likely that this will never actually be completed. Total conversion mods of old content require a great deal of effort to port and tend to take years to transition content over.
There are similar mod teams working to bring Morrowind and Oblivion over to The Elder Scrolls V: Skyrim and they've been working on those projects since 2013 at least. Black Mesa, a remake of the original Half-Life built in the Source engine, has been in development since 2004 and is just now getting an Early Access release on Steam. Game development is hard enough on its own and trying to bring forward old assets isn't much easier. So set your clocks!