Referral system is a good idea. Current problem is that every new STEEM account is given a delegation of 15 SP to start with some voting power. Steemit does not have enough SP to say, 1M users per month (i.e. 15M SP per month or, 180M SP per year). However, Steemit reduces SP delegation when a user earn some SP. If it can reach some sort of equilibrium such as return of 10M SP per month from its delegation to users or undelegate some SP on non-activity. Then some sort of referral may work.