And, as been said, if there’s good mod support, additional multiplayer modules, plus ongoing enhancement of the game through DLC and expansions, it has the potential to keep selling solidly for some time after release.
In an AMA in 2020 Sven indicated that there are currently no plans for "major DLC" as it seems like something Larian isn't interested in much in general. DLC I could see in the form of something like D:OS2 got with 'gift bags'. I could also see a handful of random smaller side quests being added as well - kind of like D:OS2 got.
But it seems unlikely that Larian develops a whole new campaign or a major expansion for the game. Though maybe if sales and demand are high enough they change their mind on that.
Either Larian or the Mod community or another developer could utilize the platform to create additional content. Its not a matter of IF to me - it's a matter of who wants to make some money. The IF to me right now is "will Larian commit to multiplayer and implement DM mode". They are really the only ones that can do that.
I know they will support the platform with additional updates, bug fixes and even content - they did that with DOS2 until halfway through 2022 - for a game that was released in 2017!
It's crazy how much more ethical you get to be when you don't have stockholders tugging on that teat!
