To make it worthwhile for SARU, any further Super Rugby expansion would presumably have to add one more South African team and also guarantee that the SARU were going to make substantially more money out of the whole thing.
South Africa's biggest issue as far as I can tell is the number of players they lose to Europe. Putting South African rugby in a position where they can keep more of their players on home soil would seem to be the one incentive that could make extra travel worthwhile for them.
Random musings -
Super rugby is viewed fundamentally different in SA than it is in Australia. And it is not really clear what exactly is best for rugby in South Africa.
South Africa actually does quite well at keeping players playing in South Africa. I can only think of Pienaar, Francios Steyn and Francois Louw who has gone overseas recently despite their franchises being desperate to keep them.
Obviously it would be great to have more money, but given a choice it would be best if this money is generated locally, instead of getting hooked on cheap dollars and yen.
South Africa needs more opportunities for players to play in a professional competition, but that does not necessarily means having more super rugby teams. Most people would prefer to have a 8 team currie cup season where players, but that would require the super season to be cut back.
Expansion into Asia and the US offers nothing that South Africa wants and if anything leaves us with less rugby that we care about.
An alternative would be for Australian teams to get involved with Japanese teams while SA and NZ are playing their third tier comps.