I guess you could make loans only for players under the age of 22. Allow loans only to be made to foreign countries(countries other than your own). Set the limit for a loan to be a season and a individual can only be loaned once(or maybe twice) in his stay for a club.(This would reset if the player was sold to another club.) That would create a decent loaning system that wouldn't make cheating easier or ruin the TL.
Check the Suggestions they are important