Great idea, here you should check this article out that has a lot more research on the benfits and how to implement a referal plan. https://blog.loginradius.com/2015/06/referral-programs-2/ . I think steem's referal plan should not be directly money as I have noticed a lot of user join thinking they will make money quickly. Only to get bored and leave. If they do implement one I believe it should be making a percent of that person's usage. If anyone else want to discuss options feel free to reply. This is a very good idea!