I want to design a unofficial map for ticket to ride. Is there a known formula how to create one, basic ground rules on what makes a playable board? How many cities is playable? What percentage of grey vs colored tracks? How many total possible train positions is too few, and too many?
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.