We are a CPanel partner noc provider with over 100 cpanel servers and hundreds of windows servers with various other control panels such as plesk, enkompass, websitepanel, helm and more.
Our windows servers run on Smartermail enterprise edition which is probably the best email system in the world.
Linux is amazing for website hosting but not so much for mail. It would be amazing if WHM could give us the ability to have it create all email accounts on a remote server.
Smartermail is a Windows server software, Its API is amazing and works with pretty much anything, so panel wouldn't need to be installed on the server at all.
CPanel could use the API to manage account creation as well as diskspace calculation/bandwidth stats etc. Their is already an open source WHMCS module for smartermail to see how easy the API is.
By having a remote smartermail server, this would release the load of the the CPanel server by not hosting the mail on the local server, it would also give cpanel users all of the enterprise level services that smartermail has to offer.