With X.com it seems more likely to me that Elon is intending to build up from PayPal, rather than down from retail banking.
The next step after PayPal is a Visa/Mastercard credit card which typically backs the PayPal account.
Each customer makes a monthly settlement with their Visa/Mastercard account via the regular banking system.
Visa/Mastercard make money via charging customers interest on monthly balances, and by charging merchants a fee.
To take market share from Visa/Mastercard X.com would need to charge lower interest rates and lower fees.
If X.com has a full banking license, customers can get their salary or customer payments deposited into a X.com bank account.
For a merchant the X.com account can settle periodically with an external bank account if required.
Property and Business loans require staff, specialist skills and a lot of paperwork, they are higher risk transactions for large sums of money and exposed the the economic cycle.
Any loans for Tesla products, cars, solar, Powerwall are one step up from credit card loans, and repossession of the car is always possible,
Exposure to any one customer can be limited by setting a credit limit, or denying a loan, if necessary.
Where Twitter might come in useful is:-
- Having a team of software developers and some equipment that can be repurposed to support this task,
- Having a pool of users who can be displayed advertisements/options for X.com services as needed.
My hunch now is most of Elon's interest in Twitter is due to the potential of 2. above. Elon doesn't like to pay others for advertising, but advertising on a platform he owns is a completely different matter.
Elon may or may not be intending to extend X.com to crypto currencies providing a way for crypto transactions to handled seamlessly. There are some questions as to how that could work, but if he is intending to do it, Elon would have a plan.