This topic is often bought up on sites like the roar. Lots of different ideas have been suggested in regards to super rugby expansion.
At the moment I think the most likely outcome will be a s18 with the kings, a central nz team, and a combined pacific island team playing in the Aus conference.
We don't have the depth to field 6 good teams, and we don't have the money to achieve artificial depth through buying foreign players like they do in the north.
Another idea suggest was breaking the comp into 2 conferences of 10 with an even 5/5 Australian New Zealand split in one conference, and a 6\4 split between South Africa and Argentina in the other. Then have a combined finals series (Top 8?). On first glance it seems this would hurt SA rugby more than it would Australian or NZ. But i'm not totally adverse to the idea.