Flexibility to employees.
More autonomy to choose and join projects.
I left due to migration/relocation. They should be more willing to assist employees to relocate by helping to find placement in the employee's desired city. After spending so much on employee training and development, they should certainly expend an equal effort to retain their employees.
Please make sure work life balance is important. Work is as important as someone family/life irrespective they are married or not staying away from family or not.
Understand the need of the employee and give suggestions after looking into the reason for resignation.