There is not a real necessity for a riser, open treads are a frequent look done in contemporary or modern designs. Two caveats...first, the tread material must be strong enough (usually thick enough) to support weight at the middle without the assistance of the riser. Second, the stair codes still apply, just like with handrails...a 4" sphere may not slip through the riser area (baby heads). There are some modern looks that comply with the codes and don't add a lot of dreck to the sleekness of the design...a stainless steel rod from stringer to stringer cutting the riser open area in half or thereabouts, an attached bar or trim on the underside of the tread that just makes the 4" minimum rule...