Have you ever all GK got injury?
Or got punished 0:3 for fewer than 11 legal player for a game?
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I have a basic idea of a player loan system:
Introducing a loan market to allow to list players to be loan to earn some money.
Increase Max year of a player contract from 3 year to 5 years (for example).
Increase Max player number of a squad from 22 to 25 (for example).
When the player of his club owner lists that player to loan market, loan applicants should bid a sum that will pay player of his club owner.
Loan applicants should deduct 1 token (for example) to Nordeus for loaning 3-star player.
Loan applicants should deduct 3 token (for example) to Nordeus for loaning 4-star player.
Loan applicants should deduct 5 token (for example) to Nordeus for loaning 5-star player.
(Token can avoid cheating and also can make game developer make income, hehe.)
But please do not make it "Loaning player more expensive than Buying player, no matter amount of money or token". It doesn't make sense.
Say loaning fee for a game season should not exceed 30% of player market price, for example.
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I do not know whether player loan system make the game better (or it destroy the game), but in real life football clubs really have loan contracts.