I like the referral idea.
To prevent cheating I thought about adding different levels or stages of referrals.
Stage 1:
Refer a friend x1
Refer a friend x2
Refer a friend x5
Refer a friend x10
Stage 2:
Referred friend reaches a total experience of 10,000,000 or a total level of 500 x1
Referred friend reaches a total experience of 10,000,000 or a total level of 500 x2
Referred friend reaches a total experience of 10,000,000 or a total level of 500 x5
Referred friend reaches a total experience of 10,000,000 or a total level of 500 x10
Stage 3:
Referred friend reaches a total experience of 50,000,000 or a total level of 1,000 x1
Referred friend reaches a total experience of 50,000,000 or a total level of 1,000 x2
Referred friend reaches a total experience of 50,000,000 or a total level of 1,000 x5
Referred friend reaches a total experience of 50,000,000 or a total level of 1,000 x10
At first I thought about total levels as well. But then I rememberd that Emps-Scape used to have alot of pures and pkers in general. If OSES is as promising as Emps-Scape was, then there should be many pkers as well. With this suggestion it does not matter whether you train a pure or a main. You will reach your goal, either by hitting the required total level or the required total experience.
The rewards will be based on stages as well. Stage 1 rewards will be low because of how easy it is to cheat your way through. Stage 2 rewards will be better and stage 3 will be the best obviously.