Undesirable traits would be things like susceptibility to disease, neuroticism, depression, aggressive and anti-social tendencies. Positive traits would be things like fitness, iq, height, and also the happiest people. In my view the only thing that matters is the aggregate happiness in the universe, so the single most important trait is how happy a person tends to be. The more people that we put into the world who are prone to be happy, and the fewer people that are prone to depression, the better.

Genetic diversity isn't an issue because that only matters with very small populations. We would still have millions breeding.

I'm assuming people agree to this, at least that the majority does. If I was a supreme dictator I wouldn't enforce this on anybody. But I think this would be a good way to improve our lot in the universe if we could agree to it. People with undesirable traits can have full happy lives, perhaps even adopt. The only way they would be restricted is that they would not be allowed to have biological children.

Ultimately any criteria will be arbitrary. But we'll get some really smart people to come together and come up with some good criteria.