I don't know if you're still interested in changing/tweaking the referral system (and adding rewards), XBTcontracts, but I just came up with another way you could do it.

Your "shares" are equal to the amount you deposited, * your multiplier (which would start at 1).  For example, having a signature on Bitcointalk might add .1 to your multiplier (and there would be other ways to increase your multiplier).  This would let you do signature campaigns and other similar programs.

And then you would get 5% of each of your referrers shares added to your shares.  

What do you think of this?  Just some ideas I came up with.  I don't know if those numbers would be good or not.