After RDR2's huge financial and critical succes, RDR3 will most likely happen. But I was wondering that where can they go next or how can they "raise the stakes"? 

From graphical/technological point of view it is going to be almost immpossible for them to improve significantly from RDR2. It would be quite pointless to make a similar game to RDR2, but just with different story and slightly better graphics. So that means that they would have to change the setting. But atleast in my opinion it's not going to be easy (unless they abandon the western setting altogether).

RDR2 map is one of the best/the best ever made. It also has pretty much every vegetation zone/environment that there is in USA/Canada: coniferous (pine and fir) and leaved forests, grass plains, mountains, swamps, semi-desert and small and big towns. So the map is going to be pretty similar if RDR3 takes place in USA. They could add rain forests if they will include Central America, but that wouldn't fit that well with western setting. And USA is the only option for a western game.

So that would leave only the time-period as a differentiating aspect. But something like 1800-1850 to my understanding wouldn't be that different from 1899. Maybe they could go as far as 1700-century, and you would be fighting against the Brits. Or maybe the Civil War in around 1850 would be different enough.

But personally my favourite option would be if they put the Indians in the center stage, because that aspect was so minor atleast in RDR1. And it could take place either during 1800 or 1700, and you could even play as a Indian. I would vote for that.

What kind of setting you would like to have in RDR3?

I never finished the story of Red Dead Redemption and I forgot a lot of it already but, if I remember correctly, Red Dead Redemption 2 completes 1 pretty well, right? The only thing I could see is yet another prequel ... maybe one that ends up to the Blackwater incident, but I have a feeling that'd be too repetitive and there might not be enough there.

I wouldn't mind if the franchise just ended or went on hiatus.

You could do a Western game in Central America I feel, by the way.

I'm thinking it will be with John during the year he left the gang

They went back in time for number 2 so they might as well keep doing that

SPACE!

If they make Indian (native american) setting they could also concentrate more on melee combat, which would make the combat more varied. They could choose the Indian wars during 1800-century or the England-France war during 1700-century that took place in the northeastern parts of US and Canada and included different Indian tripes on both sides. The Last of the Mohicans -movie would be a good source material if they choose the latter one.

Where younger a Hosea and Dutch, and even younger Arthur and John, with a different gang timeline.