Varnish-cache integration in imscp installer was a very good thing when its possible.
Radicaly decrease server load because memory is very good and fast part of a machine.
I have a Varnish How-To in another section. It will need updating after the soon-to-be change of the admin panel port, but works. You need a good working knowledge of Varnish to implement it successfully.
I don't know how to turn it into a plug-in yet
Also what makes it difficult for having i-MSCP controlling any aspect of it are the custom .vcl settings necessary per domain, but is possible with a big chunk of work.
That would probably be the initial implementation, just having a basic configuration of caching, and being able to add a domain. Later on, add feature of clients being able to control custom vcl files.
Backup system revamp I think is more important than this for now.