How I Think Ghost Babies are Made:
(Copied from this post)
I like to imagine it to be more of a arcane-style ritual where they pour a ton of their ecto energy into the creation of their offspring. If they don't have enough energy to pull it off they'll fade, so the process of making a child is not undertaken lightly. I don't think they should have much influence on how the kid(s) come out.
This ritual can be done by an individual up to like a dozen separate entities. The more participating the less energy needed to be put into it by each individual, so the more ghosts involved the safer.
There are a lot of variables involved in the process, so there's no way to predict what the kid(s) will be like besides the fact they will resemble their parent(s) in form and appearance, as well as core type. Creating more than one kid during the rite is rare, but not unheard of. Children will usually resemble what a newborn of the parents' original species, upon completion of the ritual.
This process is exhausting for all parties. So most wanting their own spawn will pick a safe location to do this rite in, such as one of the parents' lairs.
Magic practicing living beings can participate as well, substituting ecto energy for raw magic. The caster in question would need to have very large magic reserves, or have a way to store enough magic and access it during the ritual. Either way, the child will be a full ghost.
Though one could use a altered fertility spell combined with the ritual to make a first generation born halfa.
Theoretically a sufficiently powerful preexisting (born) halfa could infuse their ecto energy into their egg/sperm cell(s) during intercourse. A resulting embryo would develop into a quarter-ghost/living being hybrid, if it manages to survive the pregnacy. (Whether or not a created halfa could do the same depends on how functioning you headcanon their reproductive cells and/or organs to be.)