La Plata is very close to being confirmed the lattest venue with the Buenos Aires province city being prefered to Mar del Plata which hosted the 2011 Sevens World Cup.
http://rugbyworldcup-argentina2023.blogspot.com.br/2012/03/la-plata-to-host-leg-of-irb-world.html
With Argentina on board there will be 10 legs now. i.e.
Australia
UAE
South Africa
New Zealand
USA
Hong Kong
Japan
Scotland
England
I see no reason why it can´t be a lot larger. It is run from November-May but has, at times, huge gaps. There are no events at all in the month of January. Hong Kong is the only event in March and Tokyo the only one in April. The Gold Coast is November´s only leg too. I think two events per month is attanable pretty easily. Since it is 7 months it can be 14 legs. I´d move the Wellington Sevens to January and have the Australian event then too. This would free up two slots in November.
Aside from La Plata the four others to be added cuold well be some of the following:
Toronto which did really well for Canada vs USA last August.
Several years ago
Hannover, Germany was a good host for the FIRA Sevens, getting really strong crowds.
Cardiff or
Swansea. Rugby in Wales is and always will be well supported. Better options than the small Glasgow venue being used this year.If Wales think the Milleniunm to to big then utilize the Liberty Stadium which hardly hosts anything beyond Ospreys games.
Rio de Janeiro. The Brazilian city has applied for a leg but La Plata will be the winner. Massive growth going on here. 2016 Olympics coming to town.
Lisbon. There is no reason why Portugal cannot utilize a soccer stadium and do well.
Madrid. Same as Lisbon. I´d prefer both to Italy.
Lyon. A return to France is always an option but a venue other than Paris would be my preference. I say Lyon because it is France´s second largest city, the region produces plenty of French players and is underused by the FFR.
Moscow. Will host the 2013 World Cup. Pretty clear that it makes sense to host a leg.
Punta del Este. The Uruguayan resort city hosts a good event every year featuring South American teams.
There is also the possibility of an existing country hosting another leg. Dunedin could host for instance as could an eastern USA city like Boston.
In my view the four added venues, aside from La Plata, should be Lyon, Hannover, Moscow and Toronto.