Great idea...
What can easily be done is the one 3.5.0 existing hack where if there is a referral the field isn't displayed so the registering user can't wipe out the referer (
https://vborg.vbsupport.ru/showthread.php?t=98592).
On top of this a quick code in their User CP or something that just tells them how many referrals they've had and the URL to send people to in order to have them counted as a referal. Maybe have it go several levels deep for extra interesting statistics.
Add in the IP checker just to make sure that people aren't registering fake accounts to be a top referer.
Create a page that just lists users with high referal numbers.
Allow users to view who they've refered. On/Off option for this in admincp.
Ok, who wants to take care of this?