To my knowledge, a Fiddler extension has no means of getting the raw TLS handshake data.
This would be extremely useful for some extensions I develop. Two use cases are, first, getting the TLS extensions. The primary motivation for this is to extract SCTs from the signed_certificate_timestamp extension. But I am sure there are other cases where I'll need more.
The second being getting the certificate in the case of a blind tunnel when HTTPS interception is disabled, then the certificate chain could still be obtained from the ServerHello.