Multi server management

You'll need to be more specific on the features you're after. It's possible to launch multiple instances of RemoteBox to connect to different servers simultaneously.

Dear Thank you for this suggestion. I will

I possibly understand that there would be possible to run VBOXWEB service with any user who would be possible to authenticate at host. There could be also any SSO (single sign-on) service used at host, not only local users.

I would say that this is the feature I expect also. I would suggest to offer two authentication modes:
single user authentication like currently is (VBOXWEB_USER at /etc/default/virtualbox)
multiple user authentication - all users who has been created locally or able to log in using any SSO accepted at host

Currently also another problem already with single user environment is missing jail feature. It means to lock user into home folder - this would be configurable. But this seems and has been reported as another issue.