So far as legal implications are concerned, you can not with hold the certificates of any except for the purposes of verification.
Secondly, there may be major loopholes in the working system you need to look in to....
As somebody has already mentioned, you may mention the minimum number of years of employment and compensation......
Try to include the critical termination clause, like minimum period, until the company gets another employee in the place, with hold of salary during final settlement, legal actions, and written letter for the reasons of resignation pertaining to workplace and justification.....
there may be salary issues or relationship issues, or the work weightage...... imbalanced financial investment.....
you need to analyse in depth and try to resolve......
Wish you good luck.......