If wagon speed was affected by the quality of the materials used in it's construction and the bull pulling it, wagon racing would make for excellent competition.
It would make for a fun competitive gaming experience, and kick up the game world economy. We can gamble on the races, rent out our custom built tracks, and hire ourselves out as track workers or pit crew.
Somewhat relatedly, a gate capable of letting wagons through would be nice, especially if this happens and we want to charge admission to the tracks we've built.