You do not have to wait until custody is decided to remarry or live together. You do not even have to wait on Equitable Distribution is decided. The only thing you must wait on is Absolut Divorce.
There are two ways in which a court could look at this change of circumstance. The positive: You are in a stable relationship and will have a sibling for your current child/children. There will be two adults in the home to supervise and care for the child/children.
The negative: You are starting a new family and with a new child may have less time to devote to your current children/child. I’m NOT suggesting this is the case, but this is how it could be seen.
I’ve always said that as long as the child is not being treated badly and it’s an option, the best solution to custody is joint custody with equal time. I realize that this does not work for all families but it is the most fair for the children involved. They love both parents equally and should never be made to choose. If your ex is rational, which may not be the case if your in a custody battle, it may be a good idea to discuss this as a solution. If you can get along with him well enough to discuss the children and leave your own issues out of it, it truly is the best solution for the children.