Your going to need a bank who values the asset as collateral. Lets say you have 10 BTC and you buy a house for 400k. You would transfer 4 BTC into a joint wallet with the bank. Neither party could move the coins without the other key. An interest rate would be determined and you would make monthly payment to the bank. When bank receives payment their is a normal mortgage. Each month a portion of your bitcoin would be released back to you. The value of the bitcoin would not matter. Even if the price goes down the bank does not force you to reup. The fiat value of the bitcoin is mot part of the deal. That is a rambled idea but i think there is something to it