Is money the only factor in signing a Free Agent? Also, I think it would be dope if you could sign a player by guaranteeing that they start. Like once you sign them you can't even move them on the depth chart.
Currently, yes. But I do have near the top of the list a relationship history between players and owners - if you do actions that cause the player to be unhappy, such as bench them/don't start them, cut them, etc. they will eventually become angry enough that they will not re-sign with you at any price. The higher-rated the player, the more this effect would have.
