Well Red Dead Revolver is 'In a separate universe.' from Redemption according to Dan Houser; it's only mentioned as a sort of myth or story in the Red Dead Redemption games. I also can't see Red joining a outlaw gang even if it was a 'noble ' type, considering his real only concerns in Revolver is avenging his parents; the only thing he does to directly help people is giving away Griff's money, although this is probably just because he doesn't need it.

Maybe Red Harlow can team up with Niko Bellic in GTA 6

Agreed with Mothman. Sorry, but I can’t see it working.

I've said this in another chat, but I think the future of RDR should focus precisely on that: the future of the universe. Imagine riding around the Midwest during the 1930s, when horses began to fade away as automobile-mania took America by storm. A game where the protagonist plays a John Dillinger-esque, Depression-era bank robber riding with a gang of fellow outlaws, robbing banks, perhaps bootlegging, and evading the increasingly-powerful law could make for a perfect transition out of the Wild West and into the modern era, which would epitomize what RDR is about: the fall of the West and the end of a legendary era. It might have the same feel as RDR 2: being hunted down as civilization takes over, but it would make for an interesting setting, as there could be a mixture of folk and jazz in the soundtrack, multiplayer could focus on bootlegging and bank heists instead of moonshining and coach robbing, and the Tommy Gun would be the quintessential weapon of the robbers and the police. It might not be your conventional Western game, but it would make a heck of an RDR game to sort of bridge the themes of RDR and GTA together in what could be a wildly successful genre "crossover" of sorts.

I wish Rockstar would stop screwing around just say that GTA and RDR are the same universe. They might as well be and they could do really cool things with that. 

Imagine having a GTA and RDR crossover with two playable characters. One character is living in modern times and the other character is living in the 1800s and when you switch between them it’s the same exact map but different time period. Also the two characters stories must be intersect some how, maybe they could be descendants of the same crime family.

The thing with the 1930’s is by that time most of the west was already settled and RDR 1 already shows that

I agree that RDR 1 already shows the West's settlement, but note that the epilogue - set in 1914 - is still hardly different from the situation in 1911. The 1920s-30s setting would certainly show a West/Midwest where civilization has taken over, but has the rule of law come with it? I get that a Great Depression-themed game might not be one's typical Western, but it would be a perfect transition from the RDR era (genre-wise) to the GTA era, and it would show the last gasp of lawlessness west of the Mississippi. In a way, John Dillinger, Pretty Boy Floyd, et. al. were the last true outlaws.