I'd like that, too. But as I understand it, anything like this they implement might have to come from the founders' stakes or some software layer, since the main code is already baked in. Another way to look at Steemit, though, is that the whole thing already is a giant referral reward mechanism. If you have lots of Steem today, your money is worth more because the price is increasing. If you have a blog post that's good, you can promote it on the Internet and get paid for anyone who comes in and upvotes it. If you want to tell others about the moneymaking opportunity of posting or voting, get them here and they will thank you with upvotes. Even if those aren't going to you directly, every new person is adding and curating content + maybe bringing in money, hopefully power ups. So it is kind of a referral system already is you believe that we all benefit from growth.
EDIT: It seems they may be considering some incentives for referral signups after July 4. We will see...
RE: Why Steem price is rocketing but volume is low