What are the subfields of cryptovirology ? On wikipedia I can only find one subfield : Kleptography
Are there other subfields and are they already all defined ?
Are you just looking for an enumerated list of topics? Ransomware tops the list today. But there are other malicious attacks that involve cryptography, including evading cryptography (such as protocol-downgrade attacks), exploiting asymmetric cryptography, deliberately introduced information-leaking side channels (aka kleptography), and the use of poor or questionable encryption (especially when a more trustworthy alternative may exist.)
If you haven't already, read Malicious Cryptography: Exposing Cryptovirology by Young and Yung. It was the first thesis specifically exploring the topic of cryptography-based malware. The authors maintain a site at http://www.cryptovirology.com, which is more up to date. It has a lot more information that is probably better suited to answer your specific questions.