In the end is making whatever makes you happy. But that's one of those recurring answers you get.
In my opinion, a game should have some setting (something that can fit in a couple of pages or even one: nothing more shoulbreaking than 50 pages of background before even explaining the attributes), a few options for creating characters (something manageable but enough to make sure nothing repeats too much), system, enemies, some adventures and, possibly, maps. Keeping the core small allows more people to read it and those really interested will go for the expansions.
Everything else, for expansions. Even those, sometimes just add mud. Pre-made adventures tend to be fairly popular.