Ability to renew a subscription based on the subscription plan's configured subscription expiry date.

So if a subscription expires in ten days from now and it's subscription plan is set to expire in one year, I would like to be able to click a Renew button to extend the subscription with one more expiry unit (1 year in this case, 2 years if the subscription plan would have been set to expire every two years).

Also show renewal date in the list and sort by renewal date

There is relatively new anti-spam tool - Rspamd. I think, it worth to be integrated with Plesk as an alternative to SpamAssassin. It shows better performance as the author says here http://rspamd.com/misc/2016/03/03/rspamd-performance.html

The ability to empty a users emailbox without deleting the account and recreating would be great.

A carddav and caldav Server

I want to be able to set the recipient_delimiter. But, currently, any customizations to main.cf get overwritten.

Currently CloudFlare ServerShield is implemented using partial (CNAME) setup. What is the point in a partial setup when you can benefit from full DNS? You get anycast DNS with CloudFlare on full DNS.

My biggest dislike ist only FTP_SSL ist supported

In 12.5 cannot find a way of adjusting php-fpm settings for a domain through the panel. For example:

http://php.net/manual/en/install.fpm.configuration.php#request-slowlog-timeout

See the discussion, which got nowhere, here:
http://talk.plesk.com/threads/adjust-php-fpm-settings-in-plesk-12-5.336206/

imagine you get a domain with thousands of dns records and you have to create each by your self in plesk!!!

When mailboxes on a domain get full, it would be very useful to be able to purge them. This is especially relevant when all mail in the mailbox has been downloaded to an email client since the only way to delete the mail from the server is to delete the mailbox and recreate it.

SPF information is so far entered through the TXT record. Adding the emerging new SPF record would make sense.

In Fail2ban (great idea to include it in plesk!) settings you can set "Time interval for detection of subsequent attacks" (findtime) in general. But it would be interesting this setting per Jail.
Why?
you could have 2 jail with same filter but different findtime. Example:
Jail 1) 5 failures in 600 seconds: 1800 seconds ban
Jail 2) 30 failures in 86400 seconds: 604800 seconds ban

There are bots that detect if you have some protection fail2ban or similar and it will adapt, login attempt every 300 seconds for example. Jail 1 no detect this attack, but Jail 2 yes.

Many times I find that some subscriptions won't sync because they are locked. I'd really like to know why they are locked. So far, I've been able to unlock & sync with no noticeable changes. But I'm left wondering what caused the lock in the first place. And I'm afraid that I'll realize later something no longer works as expected.

Allow the ability to assign separate SSL certificates for multiple mail servers.

Have the ability to provide more Nginx builds/packages. For example, offer a 'nginx-full'-package with more modules.

For example, I need the Nginx headers more module. Since this need to be built-in at compile time, it isn't possible to add this later on. So that would mean replace the Plesk version of Nginx with the distro-default. (Heck, why does Plesk need it's own Nginx-build, anyway?)

No, passing headers from Apache is not an option, especially when I'm using the Nginx - PHP-FPM setup.