For instance, one of the most popular groups is rec.games.frp.misc. Games, sports, hobbies. Often, pure babble. Anything that doesn't fit in the above six categories. Anybody with enough Deep Net Knowledge can create an alt group.
New ones appear all the time.
Usually they get a few postings and then languish, but some last indefinitely. Check out alt.conspiracy , or alt.purple.dinosaur.die.die.die . Or alt.pave.the.earth . And there are local newsgroups. Last but not least, there are system-specific newsgroups, intended only for the users of that system.