There is a forum on Mirable Visu for people working together to try to get published in Outercast. I signed up for it and the theme is Brave New World (like the book).
My idea is that there is a spabitat (space habitat) in the future where they use genetic engineering to put people into castes. If you have an unregistered child, then that child doesn't get genetically screened or engineered. That child is then an outercast and ineligible for government aid or citizenship. The people are happy and well adjusted because they are all doing jobs that they are adept at.
The purpose of the castes is only to give you an advantage in a certain profession. There are no restrictions on a social level. People live in long houses that have a sort of extended family in them. Everyone in the family could be a different caste.
When you apply to have a child you can choose the caste to which that child will belong (something like having one child who is a jock and one who is a brain...).
The story will be about a peacekeeping force from this world (spabitats are called worlds) is sent to another world based of a request from that world's government. It includes an ambassador and a military contingent. The regimental XO falls in love with a local national from the second world. Because she is an outworlder she is automatically an outercast and unable to have registered children. Therein lies the conflict for the protagonist.