I would like to see these Merak related features added in future versions of Plesk. They are all supported by Merak so integration with Plesk shouldn't be too much trouble.

Mailing lists:
* Add subject prefix
* Set reply-to

Email accounts:
* Enable IMAP

Server settings:
* Allow relay per IP address
* Backup of configuration

Also it would be nice to be able to uninstall Mail Enable once Merak is up and running.

Also I'm very unsure wether spamassassin and anti virus functions in Plesk currently really are integrated with Merak.