I understand this. However, designing a good REP system is difficult, designing one that cannot be games even more difficult and designing one that cannot be gamed and users don't sell their accounts with REP is almost impossible.

And if you achieve it, making it SP * REP is probably not a good idea because it takes power away from the stakeholders.

thus, it would make more sense as a minimum safeguard to become a witness.
(Need REP > X to participate in consensus).