Overall, it’s a great app and I love it sooo much. However, it think it would be cool if you could trade some players on your team for in-game cash to spend on new players, kind of like setting up your own auctions. This only a suggestion though, so if you are thinking of getting this game then I highly suggest it!
Edit: I think you should be able to auction off players for cash. The starting price would be the price of that player in normal auctions, and levels would not reset when sold. Right now, getting cash can be hard, and with constantly changing player values, it can be necessary to replace your entire team at any given moment. Plus, releasing a plster you spent a lot of cash on can be very hard, so why not be able to get money for it?