How to design a Ticket to Ride fan expansion?
Try analyzing your Ticket to Ride game that you already have and base your first expansion on that board. Measure the board, count how many of each color card there are in the deck, and count how many tracks there are of each color. I would model your first fan expansion after the base game without any add-ons, see if your expansion works, and then try making progressively more complicated fan expansions.