I know the CloudLinux topic has come up before and I think that personally, some people here are correct in their opinions that shared as it currently is not a viable option for most service companies (PHP abuse, Apache symlinks attacks, race escalation privileges, etc).
Now, it seems a type of Jail is already being worked on and when I suggested namespaces on the forums I was told nobody is using it (but that is not a reason not to innovate) and I also mentioned cgroups.
I may say that cgroups, that CloudLinux uses is already being implemented by some of your competitors. Plesk Onyx now seems to have cgroups build in and I don't see why Virtualmin could not benefit from it either as its built right into the operating system.
As you can see it has cgroups already and unless Virtualmin wants to be outpaced and fast I would SERIOUSLY, and I mean like if you really want to compete and stay relevant on offering an option to use resource limits with Virtualmin plans. It's a must requirement for shared providers. And that would be the right step in the future as well once it can play nice with a jail kit type of systems. Is is perfect? No, but is it stable? Yes, it's built into the OS for years now. Does it solve real problems? Absolutely! Is it hard to implement? No, not really. But it would be a pain to manage on multiple servers, so it should be done from VM. It would require a bit of change on the limits of the plans and if you want to build a resource manager where users can actually see their stats or consumption from their own panel, well that would be AWESOME and would put Virtualmin on a whole new level when it comes to hosting control panels.
I know you have Cloudmin but I didn't suggest this there because this is a feature in particular useful for shared type Apache virtual plans that run PHP and scripts. It has nothing to do with virtualization but just having a bit more control on accounts computing abuse on a server that has many websites hosted.
If someone wants my help on this, I'm happy to chip in, as I'm actually working on the feature regardless if you plan to add this or not. I'm just suggesting this here because common sense was that nobody else was doing it and that link proves some developers here wrong. Plesk did. At least Plesk Onyx is a nice surprise if you ask me since Plesk was sold they are doing quite interesting and nice things with their software. I would still not use them based on my previous year's bad experiences but they seem to be innovating a lot lately, including Docker support (which I also suggest here, to be able to launch a Docker app from Virtualmin but that actually runs on Cloudmin). This means, users can manage Docker from Virtualmin, but they, in reality, are managed from Cloudmin, Virtualmin is just a gateway to it for a specific app like Ruby, NodeJS, etc. that requires its own environment and can run on a standard shared account.
I know this may require some work but I really want to move the project forwards that direction. You have to start thinking above just deploying a regular Apache vhosts. It does not work in a multi-tenant environment anymore today unless there are a more strict resource and security control on accounts. This is something that users WOULD absolutely pay so it would be a nice welcome addition to the PRO edition. Why not? Vs paying Cloudlinux and just buying the PRO that has similar features, it's a no-brainer if you ask me. If this is done correctly it could boost Virtualmin sales.