As a supporter of a non league side in England, this is a big one.
When a side turns professional, there are a number of players who will then leave the club because they want to carry on as a semi professional due to other work commitments. Hence they will reject the offer of a professional deal and only look for semi professional contracts. However if they do want to turn professional, they need to require a much higher wage to cover what they were earning at both jobs.
To add realism I think that this would be a great feature.
Buxton in the National league north are currently going through this process with the majority of their squad leaving as they don't want to turn professional.