How can you gamify the recruitment of new employees?
I would try to work on stimulating rejected candidates to become referrals. 

Every candidate who has been rejected is already quite well into what the job is all about. S/he should be very good at explaining the job requirements. (The pitfall of the majority of job ads, IMHO is that they bitterly fail at explaining what the job is about.) 

At the same time rejected candidates must be likely to know colleagues good for that job. And it is in their self-interest to promote those colleagues, because they would have less competition for all the other vacancies.