What to model next?

I don't.
Looking at how pricing is going in this market can only inspire you to find something else to do.

When in doubt, try to find something to model, but something on what you can learn some new thing and improve your skill. I always find it best to do when no idea, learning and selling at same time is great ;)

Both great answers but I tend to lean towards Trimitek's theory. What is trending in the market is what is selling at the moment. You may want to look at trends outside modelling as well. For instance, when Game of Thrones was all the rage, swords sold more! Outside influences like movies/ series/ toys change the way people think, and that includes game devs and app coders.

Next for me it is photogrammetry. Real objects became 3d models. Is not it interesting?

maybe search for trending models and model what you find interesting.