Assuming the .com, org and the cctlds are taken, and I would speculate on the future, I imagine meaningful extensions, such as .store and .app for commercial users and .blog and .me (from Montenegro) for non-commercial users are here to stay. As a truly generic extension my bet would be on a short, cheap, neutral and aesthetic one, such as .one. But if you want to make money, I would focus on the commercial users who want a meaningful domain.
In the last few years the ntld market hasn't really made a lot of sense, but I expect that people will slowly discover the more meaningful extensions and start to use them if they remain affordable. Which could lead to more familiarity with those ntlds, a wider adoption and a more lively market.