I like the idea of saving money for interest ; But I dont like the idea of loan from the bank.
The more you save, the more interest rate you can enjoy. This should be realistic.
As for the loan, it would destroy the game as people intend to spend all the money to improve their team until bankrupt. And then create a new team, which could not be a sustainable trend for this game.
If you really want to get loan, I am afraid that BB needs a system to estimate the value of the whole team, say your 10 players are worth $500,000 (dont argue with BB about the fairness), then you could borrow $5,000 (1% of the team value). Well, does that $5,000 help your team much?