IMO, here's what direction the story and gameplay should take. Note: These suggestions are based on how the game's story is already setup, so it should feel natural. I'll keep it simple.
What do we know so far?
- Player and crew crash land on sex island.
- Sexual temptation makes them wonder if they should ever leave; after all it's a paradise.
- Player and crew decide to stay, explore, and learn about the island.
- A social hierarchy is present on the island; example: That big rhino guy is obviously the leader.
So the player wonders...
MC: "Could I become the leader of this island, or help someone in my crew (a guy or girl) become the King or Queen of this island?"
So the player decides to either become King, or help someone in the crew become King or Queen; or similar title/premise.
A) (ALPHA) Player wants to become king of the island, naturally leading to an optional Harem, eliminating competition, or shares the women with other guys.
B) Somewhere between B & C.
C) (BETA) Become the tribe's sex slave of which all the men and women dominate him, or whatever the player is into, basically the opposite of a leader, the lowest of the low. Becomes Shey's helper or something, helping her to become the Queen of the island.
So if the player wants to rise up in the tribe, they'll do quests that help members of the tribe, and by helping them, the player earns status and rises up in the ranks. Or they'll help someone like Shey rise up in the ranks and become her servant or boyfriend, or something similar.
Finally, if they designed a system where players can create quests and share them with the community, everyone can make quests that cater to the above premise, paving the way for people to make tons of mods, like in Skyrim for example.
The above premise would not require any new areas to be added. The player just goes out and kills mobs and does fetch quests to rise in rank & status. As he rises in rank, the NPC women are like, "wow you killed four of those creatures by yourself? So impressive. *SEX ENSUES*" The player then challenges other males to duels and stuff, and when he wins he gains even more rank, until finally he's KING of the island. Simple premise.