I mean, it's worth asking, but anyone with a good niche would be smart to not post it publicly unless the benefits of publicity outweigh the risk of competition.
Most people are basically lazy and will talk a good game, but not actually do anything.
While there are many interesting ideas in this thread, I expect there are a fair amount of people lurking with much more profitable and higher leverage products. So maybe just take the thread with a grain of salt and don't think of it representative of a ceiling on what's possible to build in 2019.