The premise that the world that we are living in is the best one, that “Everything is at best in the best of worlds” is wrong.
Pangloss, the hyper-optimist created by Voltaire in Candide, is shown as being incorrect.
Not only is the scientific community quite doubting on the singularity of our world - Hawkings believed in an infinity of them -, but it is most definitely not the best it can be.
To state that everything is as good as it can be in this world is simply incorrect. Just as my own grades have room for improvement, the world does too, in matters social, economical…
Just because you may believe that this world is the best and only thing we’ve got, does not make it so. Best, by definition, signifies that there is no room for improvement. However, seeing as mass poverty, starvation and illnesses, to name a few, are still very existent in our world, there is significant place for betterment of the world.
Your premise is therefore incorrect.
As for the goal I think God missed, I believe God is responsible for the existence of evil in this world. Although God might not have created evil himself, I believe God is responsible for the existence of evil in this world. If, I repeat, He did not create evil, - many claiming that it was Man in fact that did – He could’ve very simply destroyed it, omnipotent as he is, if he is as benevolent as he is claimed to be.
Therefore, when asked what goal I think God missed, it is that of omnibenevolence. It cannot possibly be claimed that God is omnibenevolent. Not with the examples I gave previously. If he truly is omnibenevolent, and does nothing to stop the woes I examplified previously, then he either cannot do so, and is therefore not omnipotent, or refuses to, making him lose his place as omnibenevolent.
That is the goal he missed. That of making our world as good as it can be.