We'll be implementing this with multiple WP multisite installations on multiple servers. Instead of expanding a single multisite installation, we'll just add new servers / installations as needed (thus not putting all our eggs in one basket).
Since the server name appears to be the domain name of the individual multisite installation, I'm wondering if it's necessary to register a separate TLD for each installation, or if we can use a subdomain without messing up the users' site hostnames.
For example, instead of each multisite "server" being:
...etc., can we just set them up as:
Of course, we want users to have their own chosen website names, like username.ourdomain.com, plus the ability to map domains to their site.