Handling the mess that is Multisite registration and emails when you’re on a per-site basis.
When you lose your domain, you may not lose anything.
It’s a holy-grail to many. Make a per-site registration form that will not only let users sign up, but actually add them to the site they signed up from!
Domain registration is a different beast than it was a decade ago. What used to be faxes and phone calls is now all email.