This is a gargantuan topic. Can you, as they say, narrow it a bit?
I got started by just listening. When I discover a niche, I grow for it. (yes you may steal that line :) We have Ti food restaurants, so I grow kaffer lime leaves. Now I'm looking into other things they want fresh. There are lots of date growers here in the Indio area, but only 5 of us grow a variety of dates (Barhi) that can be eaten yellow fresh, instead of dried (called kahlal in Arabic sp?) I grow fig for the organic market. Actually everything I grow is organic and I look for markets that want that.
I consider what grows well in my area, and what I personally can grow well. I don't claim to grow everythin well. My wife can grow great pepper and onions, mine never do well.
I liked an idea mentioned elsewhere, one market grower in this forum, gives a seed catalogue to a chef he sells to and lets him pick something special he wants each year for his upscale restaurant.
The bigger (and more tasteless and less variety) the big growers get, the more room there is for use little growers.
Arthur Futterman the Date Palm Guy