Extrapolation

A key part of world-building in SF is extrapolating from our present-day world.  We sit down and work out what would happen if things changed in a certain way.  It's conjecturing about the future from the standpoint of what we know today.

Extrapolation turns our story setting into more than just a future London or New York.  It has to be a key part of our story, as essential as working out our plot.  You may want to use a future London or New York in your story, but. If you're writing SF those future cities will look very different from today.

Extrapolation of science or cultures from what is known at the present day allows us to make our stories plausble.  We can't throw out the science we know, we can only modify and extend it with new theories or discoveries.  We have to convince our readers that our extrapolated world is realistic.

A good way to do this is to focus on a current trend.  What will be the consequences of the rampant over-population of Earth we are presently in?  Conflicts will break out over scarce land or scarce food or water.  Will we see horrible genocide as 'ethnic cleansing' reduces humans to some idealist master-race?  Will we run out of power, or will the crisis finally force us to put the money and resources needed into developing clean energy that helps to save the Earth?

You could start from the other end too.  Imagine a future situation where, for example, children are rare and the majority of the population is ageing fast beyond the reach of rejuv drugs.  What caused this?  Plague, war, starvation?  Or has the population's fertility declined so far that only a rare few are fertile?  Is this an effect of diet or lifestyle, genetics, or something more subtle, like Gaia limiting our numbers?

Taking the time to thoroughly work through your extrapolated change in the world of your story will pay dividends.  It allows you to write an authoritative portrait of the time with believable details.  And that always has to be the aim of an SF writer.