The mysteries of the "Golden Age" featured amateur detectives who became embroiled in solving crimes accidentally, meaning they just happened to be 'on the spot' at the time the crime occurred.
In modern mystery novels, however, there seems to be a trend towards making the protagonist/amateur detective become involved with the mystery through his or her profession or hobby. One of my favorites of this type is Vicky Bliss, an assistant curator of the Munich National Museum in the series penned by Elizabeth Peters. Vicky's adventures revolve around missing antiquities or stolen art.
Recently, I began reading The Blue Rose: An English Garden Mystery by Anthony Eglin. In this novel, two amateur gardeners experience crime and mayhem when they discover blue roses growing in the garden of the home they'd just purchased.
I believe, very strongly, that one should study the current market of one's chosen genre to learn what is being published because, naturally, those are the novels that are being purchased. And it seems to me these types of novels are in much more demand than the usual "house party" mystery novels, in which the motives are purely personal.
Luckily for us, any type of profession or hobby can be used as the plot for a mystery. Below are a few suggestions.
1) A stockbroker uncovers an insider trading plot.
2) An English professor learns of a missing "Tale" written by Chaucer.
3) A fashion designer stumbles upon a gang of knock-off artists, when it's revealed her Gucci bag is a fake.
4) A homemaker finds out unscrupulous people will do anything to obtain her great-grandmother's recipes.
5) An amateur genealogist discovers an infamous branch on a family tree that someone will go to great lengths to conceal.
I appreciate these types of novels, rather than ones starring professional detectives, because they help me to imagine that ordinary people, like myself, can still achieve a little mystery and adventure in their lives.