Depends on how deep you want to go, I think. For example, if A would loan money to B, A can agree that B will pay back then send coins to B. However there is no enforcement. Maybe B can offer some collateral, so you can make an even trade/escrow. I can imagine that there may be a second blockchain that tracks loans such that one can trade/escrow the coins for a loan-record on that blockchain. Therefore, A now has proof. All of the bookkeeping, enforcing, etc. makes things complicated, but loaning is the traditional loaning that everyone knows.

Mortgages on the other hand are more interesting. These are significant sums, but more importantly, it is known that housing prices went up when higher mortgages got more accessible. So if you want to now translate a mortgage to a "loan in Bitcoin", then you run into an issue with how will housing prices respond now that the accounting tricks that made mortgages more accessible may not be equally translatable.

Not an expert, but this is what comes to mind given your question.