I should stress at the outset that this isn't an original thought. But very few are, so here it is.
Back in the day, I was a fan of psionics, but in 3e that was a whole new subsystem that had some mental load associated with it. And so one of my policies very often was to offer psionics as an option at the start of the campaign, but if nobody took one of those options then it would be removed from play - there would be no opportunity to take any psionics stuff later. (If someone did take a psionics option, any psionics option, that opened the entire toolbox for everyone from that point onwards.)
Of course, that same option could be applied to everything. And, write large, it can be readily used by the DM for world-building - if one of the PCs is a dwarf then the species and all of their trappings can be added to the world, the clans, the underground kingdoms, and so on and so forth. But if none is, those elements can be omitted to make room for something else.
In theory, that still leaves a nicely diverse world - you potentially have four major species (plus any monster types you want), you probably have arcane and divine magic in some form, and so on. It's just that the world only contains those things that the players have, through their choices, indicated that they are interested in exploring.
The only downside is that the DM can only do limited world-building before kicking off the campaign, since he doesn't know which options the players will select. But that's not necessarily the worst thing, either.