As my fellow editor, Alex Cooke, would say when I rebuff his cliche putdowns, "classics are classics for a reason." He's usually wrong, but not in this instance — classics achieve that coveted label because they resonate with many people and do not fade with time. In photography — or really any creative pursuit — that can be tremendously difficult to pull off. If you aim for what's popular at the time of taking the picture, the likelihood is, it'll age poorly. Instead, you should aim for the style of timeless portraits that are as loved today as they were decades ago.
One example of this is the painterly portrait, so called because they strongly resemble posed painted portraits. Hand-made portraits, whether paint, pencil, or something else, have been perpetually created for thousands — arguably tens of thousands — of years, to varying degrees of success. In the past few hundred years, styles have ebbed and flowed, but there are a few constants, and that is what this type of photography aims at.