I wouldn’t go as far as automatically assigning users to (random) instances, as some people might really want to use the one they’ve selected for a number of reasons. OTOH, the registration page could offer alternatives (“instead of registering here how about one of these instances, that is currently looking for more users and will have lower loads/latency”, etc.). The default could lead people to some other instance, but always with the option to stay where you are. Of course it would only suggest instances that are federated with the one you’re trying to join (if the admins removed the others there was a reason for that…) or even having admins fill in a list of “preferred instances” that could have a higher priority in the suggestions list?