Well, eternal youth and good health for all is still nasty. Overpopulation is going to be a thing. Especially if that good health can't be removed in any way. I'm still going for the second one.
New hypothetical!
You've been granted the opportunity to grant humanity one of several boons. You have also been granted the opportunity to curse humanity in some way, in exchange for choosing another boon. What would you choose, if any?
Boon of Advancement: Humanity enters a 100-year scientific golden age when you choose this, speeding up scientific progress threefold and increasing how much new and old technology spreads across the world.
Boon of Vigor: Old age ceases to be an inhibiting factor(beyond the fact that it eventually kills you). All the elderly, present and future, now retain the vigor, the vitality, the intelligence, the whatnot, of their younger selves. This doesn't prevent death by old age, but that death is now a painless occurrence that occurs in one's sleep.
Boon of Magic: The supernatural comes to the world. People who study enough can learn to use magic based on their personality. It is a long and arduous process, lasting many years of study and spiritual and mental meditation, but for some, it's worth it.
Boon of Preservation: Humanity is prevented from experiencing apocalypses from now on. Humanity will not experience any apocalypse other than the destruction of their planet via expanding star, as a result of manipulated luck. An apocalypse is defined as any event that lays waste to an entire continent or more, leaving 80% or more of the people within dead.
Boon of Potential: Particularly driven humans who express their determination in some extreme manner will receive the power you know as the Gamer. Various caps are in effect to prevent growth to extreme(read: capable of causing an apocalypse) levels, but it's still quite the power, given just how fast those with the power advance. Should they lose their determination, their power will fade gradually unless they gain their determination back, at which point the power returns in force.
Boon of Wishes: One person other than you of your choice gets an antique lamp, that, when rubbed, unveils a genie. Only the person you choose can make wishes using the genie, however. The genie offers four wishes. The first wish will be granted according to the wish maker's desires completely. The second wish will be granted utterly literally. The third wish will be granted in a horrendous, monkey's paw manner. The fourth wish will only work for 24 hours, unless it is intended to undo the effects of an earlier wish. If anybody other than you and the person you choose learns about the magical properties of the lamp, the lamp explodes. If the person you choose learns that you know the magical properties of the lamp, the lamp explodes. If the genie is asked to resurrect the dead or mind control people, the lamp explodes. If a wish is made for the genie to be freed... well, it may be a good idea, but it depends on what wish is used. Wishes cannot remove Boons or Curses, nor can they accomplish anything on the scale of a Boon or Curse. If a wish is made for more wishes, the lamp explodes.
Curse of Silence: No human can speak, present and forevermore. This also applies to singing and any method of vocalization whatsoever. This also applies to using mechanical devices to create a voice for oneself. Other methods of communication are not effected. The magic detailed in the Boon of Magic often depends on speech, but not always. Furthermore, the sense of hearing of every human being is weakened, and cannot be remedied with hearing aides.
Curse of Conflict: Humanity will never unify into a single government. The fewest governments humanity can be divided into is 4. Luck manipulation will ensure this. Furthermore, governments will constantly be at edge with each other unless they have very strong diplomatic ties, and governments cannot merge except by conquering the other.
Curse of Lycanthropy: Various strains of werecreature disease spread across the world. The strains cannot be cured(not even with magic or super tech or Potential), but it is possible to prevent the disease from taking hold if medical treatment is applied within 24 hours of receiving a werecreature bite. Those with the Gamer power from Potential can learn to exert some(but not complete) control over their were form and learn to change into it outside of the full moon.
Curse of Boon: A random boon other than the ones already chosen and the Boon of Wishes is applied to humanity for 2d40 years. At the end of that time, the boon is removed, and its removal will come as a great shock to humanity, causing devastation(but not a full-scale apocalypse). Furthermore, the boon applied via this curse will have some drawback that makes it much less of a benefit.
Curse of Incursion: Every 3 months, a portal appears somewhere on Earth and some horrible threat from another universe spills out before the portal closes. It will never be apocalyptic in scale, but it won't be good at all for the local area.
Curse of Villain: Some human will utilize the benefits of what Boons you've chosen to become a great villain, one only old age will stop. Whether the villain will be hammy or truly evil is up to the Villain. Whenever the current Villain dies, another one will arise in 10 years. The Villain will always be a significant threat to the nation he or she is in.
Curse of Sloth: Humanity, as a collective, moves more and more towards laziness. There will always be exceptions to the rule, but they will be rare.