What art subject to paint?

The answer is landscapes. According to Art Business Today, not only are landscapes the top best sellers in the UK, but also subdivisions of landscapes - local views, semi-abstract landscapes, seascapes and beach scenes, impressionistic landscapes - occupy another 4 places in the top 10. That's...er...half the subjects.
Here's the list:
1   Landscapes
2   Local views
3   Semi-abstract landscapes
4   Abstracts
5   Dogs
6   Figure studies (excluding nudes)
7   Seascapes and beach scenes
8   Wildlife
9   Impressionistic landscapes
10 Nudes.
The popularity of local views proves there is a market for amateurs and semi-professionals as well as for established artists. Unless you live in a major tourist area, it is unlikely you will find a leading artist painting your town or local countryside. So here's an opening for the talented amateur. Better a local view than a Constable. The declining popularity of old masters might also be explained by their fame: a recognizable masterpiece on your wall must be a print, whereas a lesser known work might be an original.
Abstracts seem to be a fad promoted by TV tart-up-your-home "experts", because they can find abstracts to blend with their colour schemes. This might explain why Ikea sells 3,500 copies a week of David Briggs' boring abstract Fragmented Harbour: it blends with the curtains!
Doug Hyde tops the best-selling British print artists with his teddy bears and moon-faced smiley people, which keep the kiddies happy and amuse Mum too. Jack Vettriano - shunned by the Art Establishment - remains very popular, as does the late Sir William Russell Flint RA with a nice line in tasteful 1950's style nudes as well as subtle watercolour landscapes.
What is blatantly obvious is that we Brits spurn that trendy tripe promoted as art by those pillars of the Art Establishment: Tate Modern, the BBC and the Turner Prize. As one of the leading UK art publishers puts it "you get nowhere selling Turner Prize winners". There's a man who knows his business and his tripe.