I recommend starting with pathfinding, because having fluffies move around is obviously the first step in getting them to do anything in general.
I’d also recommend joining the modular fluffy project by making anything you do as reusable as possible. Making a whole game alone is a big undertaking, but making it one piece at a time with other people has much better chances of succeeding. We’d have already had a finished breeder game years ago if every new project could have built on the foundation of the previous ones! 