Legally and ethically
Can someone make a sequel to this game without the developer's permission?
No.
Copyright issues. The project can be bought from the dev, in all cases a permission needs to be given either for free or sold. But then will the next guy be like him or worse? use the fanbase to milk more? or will he be serious but bad storyteller turning the characters to prostitues without personalities?
Other than that, there can be only fan art but without story.
It's good to consider that there is no hope for this story anymore.
Unless there is some way that I write the rest of the story because it's my job, and I know I can do better than xenorav on that matter. But Ethically I prefer not to get involved myself in such things.