Sub-server creation form and single word domain names

In #4190, we've been hashing out some confusion that a customers users have with the sub-server form. Apparently they have a habit of entering "sub" rather than "sub.domain.tld" when they want a sub-domain named account. Would there be any dangerous side effects to automagically appending the parent domain name when users enter a bare word? e.g. I enter "sub" and "sub.domain.tld" gets created.

This doesn't handle going two or more deep (sub.sub.domain.tld), but I suspect users who are getting confused by this aren't trying to create deeply nested domain names.