Bank loans to Player Characters don't work in an MMORPG system because the PC has no accountability. In real life the bank can come after you for their money so long as you are alive. In the game you can create a new character, take a maximum loan, transfer assets to another character and delete, wipe, repeat.
Such a loan would have to be tied you your account, rather than a character, since you have paid real money for that (or someone has). But this breaks the fourth wall somewhat.
An alternative is for your character to build up reputation through game play (another RL investment), and use that as collateral or trust with the loan-giver. Of course such a character is less likely to need a loan in the first place, and when you build up reputation with Minor Factions there are more lucrative missions which also somewhat negate the need for loans.