Lemme tell you something: I really like this idea: Zombia, doom, struggling people, what could be a better background for a depraved erotic story? However, if you insist on executing it in this way, I can't see this game going anywhere. First of all, cut the sandbox please, it only makes the game repetitive and boring. Then, start to make a story, a good story, and think about it, there is so much potential in such a chaotic world: factions, gangs, slavery, cult, drug, prostitution and management of the sweet farm. introduce more characters as well, and give them personalities, some are rude but kind, and some are just evil. By meeting those characters, MC can have a taste of different "paths", and later on make a choice to immerse into an exclusive path or better, you can make a story of "string of pearl", and allow players to go through all paths, apparently, the latter one would be much more difficult.
just some suggestions, wish you all the best