Please, consider to implement some sort of cache purge functionality available from Plesk web interface, because if cache TTL gets extended to match Site needs, it gets complicated, i.e. need to go over Linux Terminal to purge it. A Wordpress integration would be just super fantastic, but I understand, it is not realistic to wish it.
Thank you!

Add mcrypt module to Plesk PHP 7.2 and newer.
For previous PHP versions it's included out of the box.

Mcrypt 1.0.2 natively supports PHP 7.2+ (https://pecl.php.net/package/mcrypt/1.0.2)

It's, for example, required by Prestashop 1.7.x, which is quite popular among Plesk users.

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..

I would be a good Idea to be able to change webmail URL via Plesk, not using workaround https://support.plesk.com/hc/en-us/articles/213947325-How-to-set-up-an-access-to-webmail-over-mail-example-com

As of now, feature with 178 votes is available in Plesk Onyx 17.9 Preview only: https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/17924536-fail2ban-now-supports-ipv6-please-upgrade

It would be awesome to have this feature available on earlier versions of Plesk Onyx without the necessity to wait until Plesk Onyx 17.9 becomes stable.

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

Add an option to exclude domains / clients / resellers from a backup to the Plesk user interface.

Currently, it is possible only via command line:
http://docs.plesk.com/en-US/12.5/advanced-administration-guide-linux/backing-up-restoring-and-migrating-data/backing-up-data/specifying-data-for-backing-up.68839/

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.

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.

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.

Free version of Enterprise Litespeed, it would be nice to use lsphp+Openlitespeed+LSCache (free) within Plesk.

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.

Currently only the main user-account (=system-user) can get access via SSH. Additional users and ftp-users only have access via FTP.

Please allow setting up additional users using SSH as well.

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

A carddav and caldav Server

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/

Please Do allow to create RDNS(reverse DNS) records to the users for their mailing severs.

Many other Service providers have easy setup for that like: http://www.rackspace.com/knowledge_center/article/rackspace-cloud-essentials-creating-a-reverse-dns-record

Thanks

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.