(Vhost) subdomains for aliasdomain
The onliest reason why normal subdomains aren't possible for aliasdomains, because both are ServerAliases in vhost-config. But since Vhost-Subdomains are real vhosts, I think there is no really good reason for denying a vhost-subdomain for an alias. It Maybe a bit strange that a subdomain gets it's own vhost-config when the aliasdomain itself only is a serveralias, but it really would work.
If Vhost-Aliases would be allowed (see #2982 (closed)) it would make really sense: it should be possible to allow normal and VHost-Subdomains on such vhost aliases.
additionally it could be also allowed to create normal subdomains on normal aliasdomains -> therefor only another ServerAlias would have to be integrated in the maindomain-vhost-config. If subdomain-code would allow adding subdomains on aliasdomains, there would only be a small change (recursion for searching aliases) in apache/ngix-module.
The onliest point to think about is: should it really be allowed to create a vhost subdomain for an aliasdomain - but not because of the technic - only becaus of logical thoughts. but all other cases should be allowed:
subomain for aliasdomain subdomain for vhostaliasdomain vhostsubdomain for vhostaliasdomain
what about subdomains for subdomains? when recursion is included it should be no problem to activate such a thing