Of that we can be sure. They didn’t buy FiveM for nothing. They knew very well how much all those servers (cities) were bringing in every month. Take Cidade Alta as an example, which has already had Anitta, and Complexo, which has already had Ryan SP, not to mention many others.
I’m sure they’re already cooking up something in a multiverse style. One important detail to remember is that GTA VI Online will probably be sold separately, from what you can gather in the Rockstar page FAQ:
Does Grand Theft Auto VI have any multiplayer modes or features?
Grand Theft Auto VI is a single-player experience.
I’ve already seen some pretty solid rumors saying there’ll be something built in for you to create content. In‑game shops like in Roblox for tons of customizations, and we’re still going to see a lot of good stuff there.

Remember that pre‑orders start tomorrow, but its official release is in November, as has already been announced.
What else would you like to see in this new GTA?