I'm sure they will incorporate all sorts of transportation, and perhaps even introduce automobiles, considering the game takes place in the early 1900's. A carriage is probably nowhere near the level of sophistication I just mentioned, but I definitely think there will be carriages.
Specifically, I think if Rockstar wants to extend the longevity of their online game mode, they're going to have to add all sorts of steampunk-type vehicles and contraptions. I hate to say it, but maybe more like the technology in Will Smith's Wild Wild West? Cringey, I know, but this would be for online only.