Classic curtains tend to be simple and unfussy, and in fabrics that will not date.
Consider textured fabrics such as silks, linens and velvets, damasks.
Plain fabrics or those with simple two tone / two coloured patterns.
Headings should be kept simple too - pencil pleats, box pleats or double / triple pinch pleats work well, hung from a simple pole with a wood stain finish. Also consider a simple rectangular pelmet in the same fabric, or a contrasting plain - this plain contrast could also be introduced as a border on the leading edges of the curtains.
Keep any trimmings unfussy and simple too, or leave these off completely.