Also I saw a few comments talking about how the developers have emphasized the story in the game is over, I agree it is. But in order for the game to have any expansions with 'V' involved it must address what happens at the end of the game. I think really, it simply means that the Johnny story arch is done. It doesn't make sense for them to abandon 'V' because of all the time players invest into building the character (ignoring the story aspect, just the perspective from gained in weapons, clothing, skills, ect). So a prequal doesnt make sense because it seems for the most part V's story begins when you start playing and we talk about tying things together, its even more difficult to do this because of the life path choices. In fact I would say its literally impossible. As for Mid game expansions, these could be lore breaking and overall don't make much sense. Why would a dying 'V' go off on another adventure instead of focusing at task at hand? It doesn't make sense for that to happen. As for post main story, this is where writing magic can be done to bring everything together. So major expansions have a few routes. (This has nothing to do with small DLC addons, these will most definitely be mid game additions in the form of cars, guns, maybe new customization features, small side quests, and maybe small gang factions being added for the player to fight. Overall nothing insane. I'd say mostly cosmetic or small side quests given by existing and already known characters). So the few paths for expansion will be below:
1) V's story is done, he/she is going to die and there's nothing that can be done for them. Regardless what ending option was chosen by the player. So expansions will take place from another new characters perspective. Where we will have to start all over again building a character. This will require a solid minimum of another 50 hours of additional gameplay to be developed.
2) Prequal expansions will happen which will have to address all 3 life paths chosen in the game. Streetkid, Nomad, or Corpo. This will have to be short because they have to take time to develop each story unique to one another because there's little to no correlation between the life paths prior to the game start. So it would have to be 3 separate 20+ hour stories. (Hands down the most unlikely and I would say has a zero percent chance of happening)
3) Mid story expansion which would have to do with something similar to the braindance mechanics, where you would teleport your consciousness into another world or situation and that would make up for the fact that 'V' is dying. No time will progress or extremely minimal time will pass (few hours). A whole made up story can be branched off of this. hopefully if this does happen it doesn't take a the same turn the Farcry 5 DLC took, because that would be a nightmare.
4) Post-Main Story DLC, this IMO, makes the most sense. You continue to level your character up with additional levels being added, a new attribute and skills that can be contributed to something happening in the story. Maybe being able to control other NPC's as a mutation to your character a result of the experimental biochip from the main story. This makes the most sense and ties together with the story probably the best. This will be a new story arch with a new villain (thus the developers saying the story in game is done makes sense.)
5) A twist ending that connects all the endings. Maybe we get stuck in Mikoshi and the story follows after that. This is the worst case scenario out of all of the others, because it would mean our ending choices were pointless. But this ties together the story and would make sense for post game DLC if they want to skip connecting the ends of the game. This could be a costly choice for the writers if not done perfectly, but none the less is most definitely a possibility.
These are only a few ideas I have please do add if you can think of anything else. But I think this is really the only areas where the devs can really go with expansions. None of this is applicable to small DLC being added. That they can really go anywhere with that if they wanted too.