While pinball has been growing in the collector community, it has been declining in other markets for various reasons. I think the biggest growth can be gained by getting operators back into pinball. That will require games that have short ball times, an easy to understand goal, which is difficult to master. Hence why TNA does so well on location. Of course, it helps that the game looks cool and has a great soundtrack. If a game like TNA could be priced at $4k and earn as well as it does right now, I think a lot more operators would be into pinball.
What would be interesting is if an established manufacturer would run a second line of simpler games that match the above. Even encourage some out of the box thinking: not every game needs an Italian bottom, it may not even need a traditional cabinet. It may be possible to design a great game using less than 5 coils, but is anybody trying? If a design team would be allowed to explore new grounds, that could be interesting. They may come up with a total clunker, or with a game that revolutionizes pinball again.
Another way to grow pinball is to tap into markets that don't know pinball at all. Think, China, India, territories like that. However, to break into those markets games need to be simple. Iron Maiden or Dialed In are great games, but not so necessarily entry-level games. To enter a new market, you need a game that market can understand and grow from there. Again, simple easy to understand objectives, which are difficult to master and as such a fun challenge.