One of the major points gleaned from the leaked writer's guide is that the original vision of the LOST saga was going to be different than what was actually filmed during the series. In more a Survivor tribal vibe, there were going to be at least four other major groups on the island.
When the 815 survivors begin to settle down in the beach camp, the first signal that they are not alone is when Vincent is found with a human bite mark on his ear. This evidence would lead anyone to believe that there are savages on the island. This would immediate raise the tension level in the camp because there is a now a known threat (beyond the smoke monster) in the jungle.
Shortly thereafter, two young men would be seen in the jungle. We suspect that they are from The Others, another group of people on the island. The guide did not flesh out who the Others would be (friend or foe, scientific researchers or utopian hop-heads). But having another "organized" group on the island could lead to conflict over "scarce" resources (one of the early themes in the guide outline). There is also a potential plot point in the guide about abductions by the Others of Kate and Locke, so we can assume that this group would be hostile at some point to the 815ers.
Later, a body washes a shore and the castaways see a submarine has been beached off the coastal reef. They will launch a salvage operation as the submarine could be the means of rescue, but would quickly find out that the sub is not abandoned, by staffed by military men in unrecognizable uniforms. The question is who are these submariners? Are they friends, foes, lost souls or even time travelers?
Lastly, there is sighted a parachutist who lands on the island. When the castaways find the parachutist, all the guide says is that they find a "unique visitor." It does not say whether he/she/it is even human. Depending on who or what fell onto the island, the main story line could have multiple tangents.
The idea of give or more groups interacting on the island does lend a "game like" story engine.
The 815ers have to interact with these new people in one of two ways: fend off attacks or go seek them out (as in rescue missions).
Then, like a deck of cards, these groups could have been shuffled during the series into different alliances. Each element brings its own unique skill set to the island party. The 815ers are a diverse group, but none seem to be the skilled outdoorsman. The savages would have been long term inhabitants of the island, who know it better than anyone. The Others could be considered interlopers by the island natives. They would have brought some technology and protection to the island in order to stay. The submariners would bring military skills and weapons. The parachutist would be an open-ended wild card twist.
Each group would have its own agenda. The 815ers would first want rescue, which would be similar to the submariners. The Others would want to keep their island "secret" so they would attempt to sabotage any means of escape. And the savages would want to rid their island of everyone, possibly using their island god, the smoke monster as the means to that end.
This alternative puzzle from the writer's guide would have made it easier for writers to mesh story lines in a coherent manner. But for some reason, the writers decided to pare back the island features to that of the Others against the survivors, and the survivors against themselves. Later, when running low on plot twists, the writers added Widmore as the Others main foe. And when that was not promising to involve the main cast members, the story of Jacob's candidates for island guardianship was thrown in as a attempt to resolve everything.
Though the guide does not state this, having multiple groups compete for control and peace on the island seems to have been a given. The inference that the main characters would have to build a new society on this mysterious island was the cornerstone pitch to the network. What better conflict point would be having neighbors who can't stand you?