Leaving Bitcoin as inheritance process is more harder than leaving physical properties or fiat as inheritance. Many method has been suggested under this thread and I'll suggest the private keys should be printed and included in will. Or get an hardware wallet, keep it in a secret place like a small vault including the details, then include the hints in your will.
Yes you are right "Leaving Bitcoin as inheritance process is more harder than leaving physical properties". And will option is quite secure but still it can be stolen. I think a secure system should be design for this situation.