feature suggestion for the multi-domain plugin

I am mainly referring to this post: https://premium.wpmudev.org/forums/topic/creating-users-for-added-domains-at-blog-sign-up#post-98309

I'd like to have the option of a unique subdomain across all multi-domains, i.e.

main domain: maindomain.com
add-on domain used with multi domain plugin: maindomain.net

can the plugin be extended with an option so that when I type in test.maindomain.com is checks for the existence of this blog and if it does not exist, try all add-on domains too before saying: blog not found or redirecting to the signup page?

I would love to have UNIQUE blog/user names so that once a username is taken it is taken for all add-on domains and works for all of them too.