Hi,
I don't know if someone already suggested it, but I think that merchandising should depend also on players you train. Namely, a young player bought when he's 18 and constantly trained in your team, should give you a stronger increase of merch. Maybe smaller than the increase caused by a drafted one, but bigger than the one of a player you just bought.
I am thinking about Rondo, for example, who was drafted by the Suns and now is one of the symbols of the actual Celtics.
In order to have a parameter to use for it, for example, you could use the ratio (sumofactualskills)/(sumofskillswhenbought) (just an idea, there could be something better probably)