What's your dream expectations from a sandbox adult games?
The fundamental problem is most games have
Static Written Content.
Whether it's linear, branching choices or sandbox, you are just shifting and partitioning that Written Content around.
And the reason why Sandbox games get such a bad reputation is Sandbox games require more Content while the amount that can be produced by an Author remains limited, which means there is a shortage in routes and questlines that aren't as developed and they implement Grinding Mechanics to stem the bleeding.
The most successful Sandbox games are old and have been in development for years so that they reached the appropriate amount of Content required and players have already invested themselves into that game and characters, the successful ones at least.
Sandbox games in the initial releases are pretty much not worth playing, that means a developer would need to invest a lot more before they get any return.
What I want to see more is
Dynamic Content which comes from actual Gameplay, whether it's Business Management, Strategy, Combat, Training Sims and Progression, you can make the Content based on Gameplay Loops that are much more Dynamic and possibly Replayable.
I think the biggest potential is in World Simulation, we already have certain Genres that already work like Brothel Sims and Slavemaker Sims but there biggest flaw is there is no wider world that is simulated, you are in your own isolated bubble and once you reach Endgame and run out of Progression there isn't much point to do anything.
It would be great if you had Factions and NPCs with their own Agency that you can interact with and maybe have an Endgame Goal you are pursuing.
Like if you keep selling slaves that are trained in combat maybe that faction uses it for a rebellion.
What I like about Masters of Raana is they have this factions and a bit of world simulation in the form of daily events. Simple stuff but some of your actions do have an impact.
Probably the best example of what can be done with that kind of World Sim is in the game Shadows of Forbidden Gods and Starsector. The Systems and Mechanics aren't really that complicated to implement.
Kenshi is also intresting to look at but they don't have that much World Simulation, though there are some mods that do that.
Also it's important to understand in most Sandbox games that are not based on unlocking more Static Written Content is going to be based on unlocking Progression as a Goal so it's important how you design and pace things.
Ideally it would be great if you could make it Replayable instead as that can Recycle some of that Progression over multiple playthrough and not be that backloaded on the Endgame.
Portals of Phereon is pretty short in terms of game length but it is fairly replayable. 4X Games also have that type playthrough format.