Right now the basic implementation covers TOTP, HOTP and Yubikey methods. It would be useful to include support for Duo Security (https://duo.com/docs/duoweb). Duo Security maintains PHP libraries that can be used for development (https://github.com/duosecurity/duo_php).
Adding support for Duo Security in kolab_2fa will increase the popularity of the plugin and extend its usage to alternative ways of two-factor authentication.