Scientists predict that in a few decades, we may be able to select from a "catalogue" what our babies will be like before they are born. We could choose ethnic facial features; adult height and body shape; intelligence; which diseases they will be resistant to and much more.
What could go wrong? A lot. We could biologically design ourselves into extinction.
First of all, consider that parents may want their off-spring to be tall, slender, athletic, and cheery (blonde?). But that common portrait of the optimum human-type as promoted by popular media may not be conducive in changing environmental conditions.
Darker skin allows livability in extreme sunlight. Smaller, more compact body designs require less food during famine. The list goes on.
Genetic variation allows selected creatures to walk through the gauntlet of daily life. Dangers that hinders one individual, another individual could dodge and escape to continue the bloodline. Thus, when you roll the genetic dice, it may be best to resist the urge to tilt the table; instead, trust random mutations for filling the gene pools with many varying characteristics.
Surrender our embryos to natural selection (within limits, of course). Embrace each child's strengths and weaknesses when they emerge.
By choosing what are considered to be positive traits today, could doom the human species by limiting our adaptability to unforeseen conditions -- like drastically rising sea levels or an accidental virus that targets tall, athletic people.
Having webbed feet, a short statue and gills to breath submerged under seawater may be drastically more important than the prowess to deliver a backhand in tennis. Or dribble a basketball.
From a biological standpoint, diversity is an essential condition for the survival of humanity.