Maybe add this?

It would be a nice feature to add royalties within contracts and transfers. This could be added easily. It will also help new players grow fast and keep transfers exciting.

What happens if the player is transferred to 10 different clubs, does each het royalties?

I would say keep it inbetween two different clubs. For example, div 7 club finds a wonderkid in his youth and sells him on for so much money plus a % of profits the club they sell the player to make. The player could be sold to different clubs in the future but the creator of the player wouldnt get anything; only the current holder of the player will and the club before depending on the deal of the player.

If the devs wanted to be more complex they could add it where the original club would get so much for each time the player is sold, played, goals scored and so on.

Let me explain it better:

A small club invests money into his scouts and found a wonderkid. The club then sells the wonderkid to a bigger club for 15m with a clause of 15% of the clubs future profits made for selling the player. Say the club which purchased the player grows the wonderkid which makes him world class and then sells him on for 100m. The club they bought him from would get an additional 15m (15%) when the sale is complete. Resulting in the smaller club making 30m.

They could also add clauses like; Fee per appearance, fee after so many goals, fee after so many appearances and so on. (one step at a time )

I don't think it's a good idea