You had me at futa. (Even if it's quite a distance down the road.)
An important question: is it to be "real" futanari (both biological bits down below, capable of breeding as well as being bred), trans (feminine appearance, dongly parts, exit only), or both sprinkled throughout as player options or potential slaves?
I'm assuming it'll also be optional so not to incur the wrath of those that prefer a more vanilla flavoring.
Asking for a friend.
Good luck with this gloriously daunting project! Big shoes to fill, etc. etc.