I can't wait to see where this game goes.
I would looove to see more personal development for each character, something that brings some different attitudes to the many NPCs so they each have a different feel. When creating your own NPCs this could be determined by a simple "personality" selection where, lets say you select for example "excitable" that NPC would then be equipped with dialogue text for that personality trait. I loved getting to chat with Sofia, Shale and Willow and seeing each of their own personalities and would love to see that brought to everyone else.
I think a relationship meter would go a long way, basically you have a sort of "reputation" with each character and each time you see them on an island you can only contribute so much to it, so you need to keep finding them on different islands to build it up. Imagine if after x amount of encounters you could invite them to live with you, then that way you can have your favourite NPC at your house, which could maybe open some unique dialogue, and unique sex positions involving the furniture and house.
I also really hope to see some village expansion, NPC and building wise. So excited to see where this game goes :D