"long time listener, first time caller"
As far as DLC goes, i don't think any new protags will be added, like in gta4's TLAD and TBoGT. There's already 3 characters to keep track of and progress. With Niko there was not much to progress seeing as there was not that much customization, not much to do with cash, and no skills. So it was good to add new characters. But now with 3 protags to progress EACH separately in stats, fully custom weapons, custom cars, property, stocks, etc. it makes less sense to add even more characters. They could make a new protag on a new island, which would be cool, but very unlikely as that would basically be a new gta game lol.
It's more likely there could be 3 DLCs, each containing a more in-depth story for 1 of the protags. For example, maybe 1 dlc could be about Michael before going into witness protection. Maybe a dlc about Franklin going more in depth with gangs. And Trevor, well im sure that dlc will be unpredictable and crazy haha
Personally i would LOVE to see, as some have mentioned here, a Zombie dlc similar to RDR undead nightmare. Some people may think zombies are stale now, but thats only because of all the shit zombie games that flooded the market. GTA5 has a real opportunity to make one of the best open-world zombie experiences. I like to think theres a good chance of this happening, seeing as RDR undead was a big success, and R* is very good about learning from experience.
if youre still reading, SORRY FOR THE LONG POST