It really depends on the architecture style. A few ides are:
Craftsman: 1-2 inches in flat from each edge, shallow arc (maybe 1-2
inch rise at the center.
Antique: Sort of an Ogee curve up at each end about 3 inches in and 3
inches up at each end with a flat center.
Victorian: Stepped feature stepping out and or up with stacked molding
at the edges.
Victorian Gingerbread: Brackets and fret work.
Contemporary fret work: Oak rails with turned pickets
I think the fret work idea or something with some holes in it would be
nice to let the light play out of it a bit.