Some would argue about including Barry over the rest (considering their records and service). Its just unfortunate we never got to see more of him at test level but there is no denying his quality. I'm a big fan of Eddie Barlow, he is someone you want in your team to fight by your side but unfotunately misses out.
Besides the other options mentioned above as openers (Gary Kirsten, Herschelle Gibbs, Eddie Barlow & Bruce Mitchell) who missed out in my selection.
Other notable mentions -
Herbie Taylor (1912-1932) opened succesfully at times for South Africa but would jump between opening and the middle order just like Eric Rowan (1935-1951) did. Jackie McGlew (1951-1962) was a stone-waller opener but there is no arguing with his test career record and his stature in SA cricket. Another was all-rounder Trevor Goddard (1955-1970) who opened for the majority of his career. Lastly Jimmy Cook (FC career 1972-1995) would have eventually taken over from Eddie Barlow as Barry Richard's opening partner in the mid to late 70's and into the 80's. I would have had no doubt that he would have had a successful test career barring isolation.