Please add PHP Composer to chrooted Environments.
Many Customers Need PHP Composer to install various scripts, cms Systems and libaries.

A current big Problem is the new typo-neos System - you must use Composer to install at sources correctly

A other Problem are PHP API's and Libaries like Payment Companies like PAYMILL etc..

Add description/comment field to trusted ip address on fail2ban

It is not possible to add an ip manually to fail2ban trough Plesk interface. Sometimes you detect an offending ip address which you want to ban from your system, before it is detected by recidive rule.

phpPgAdmin support was dropped starting from Plesk Onyx 17.8
This means that Plesk does not have any management tool for Postgres databases shipped with it.

I suggest to include pgadmin in Plesk installation since the latest ve
rsion of this software (2018-09-06 - pgAdmin 4 v3.3 Released) has just been released, with adequate security. More info about it you can find here:
https://www.pgadmin.org/

It would be great to have ftp account with an expire date.
A sort of temporary ftp accounts.

This becomes really usefull when you need to share your ftp details temporary with a webdeveloper, or somebody else to maintain of check an website.

I always make a new FTP account for this sort of events, but then forget to delete them. It would be real nice if you could set a expire date that the account automaticly blocks itself after that date is past.

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.

A carddav and caldav Server

The implementation of the centralized smartermail in plesk onyx for windows is good, but it should also be possible to integrate this feature into the linux edition.

Many hosters provide both windows and linux hosting with ONE centralised mail solution. Since the integration for winodws is already done I suppose it shouldn't be too hard to integrate it into the linux version as well since smartermail is completely API based.

This would allow you to see mailboxes that are no longer in use.

Currently docker containers are skipped by the backup manager, would be great to include them as well.

There is an spam filtering option to "Move to spam folder XXXX" on Linux, but not on Windows (Plesk 11+).

Unfortunately, horde doesn't work so well and every time when a message with broken header arrives, horde displays "Invalid Address" and no subject to all inbox messages. The described fix in https://kb.plesk.com/en/121175 doesn't work in this cases, forcing us to find the broken message manually (or scripting something to do the dirty job).

This feature should exists in Windows env as in Linux.

The DMARC support in Plesk Onyx 17.5.3 ist incomplete. If an incoming email is rejected due to a DMARC fail there is no mailer daemon generated to inform the sender of this email.

Please add this important feature to DMARC support. Otherwise it would not be usable for most customers.

Implement http://www.wildfly.org/ instead of deprecated Tomcat

Created according to the comment from this article https://support.plesk.com/hc/en-us/articles/360000322173?page=1#comment_360000633734

It would be great if it would be possible to create multiple users which can use the Git repository. Today only one user can access the Git repository. It is the admin user which was created during the creation of the subdomain. New created Web Users for the subdomain can not access the Git repository. It is a feature which I am missing because I wanted to host a private Git repository for our small private team.

https://talk.plesk.com/threads/can-not-create-new-users-for-local-git-repository.342409/

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.

According to a ticket I just opened (80693) Plesk currently doesn't support a clustered MySQL server, e.g. an active/active Galera cluster.
To avoid data-loss or corruption in a server crash, I'd like to run MySQL in a Galera cluster to automatically sync the data again once Plesk's MySQL started after a crash.

We have a setup where webmail.example.com and example.com point to another server than our Plesk instance. Because of this we can't enable Let's Encrypt on Webmail since the CSR contains webmail.example.com and example.com.
So I hope you consider adding an option to only create webmail.example.com (without SAN example.com)