Ok, I think we have to balance things a bit here. On the one hand, we want to expand rugby and take it to "new markets", but on the other hand, we don't want traditional rugby nations to lose interest... I think that the IRB might start a rotation between traditional and "new" (probably not the best term, but you all get what I mean) rugby nations.
As 2015 will be in England and 2019 in Japan, I don't see 2023 going to Russia or USA/Canada. Italy and Argentina are in the limit of traditional and growing rugby nations, but I think they might get a chance in 2023 under the condition that 2027 goes to one of the old boys. If 2023 is in Italy, I think 2027 will go to South Africa (there is no wy they will let South Africa too long without hosting a RWC). If 2023 is in Argentina, 2027 may go to Italy but with many games in other countries, or to Ireland (I serioiusly think a RWC in Ireland would be AWESOME). I think these are viable future hosts:
2015 England
2019 Japan
2023 Italy
2027 South Africa
2031 Russia o USA/Canada
or, alternatively
2015 England
2019 Japan
2023 Argentina
2027 Ireland or Italy+others
2031 South Africa
2035 Russia
They may even give 2023 to South Africa (booooring!), in which case anything can happen afterwards
2015 England
2019 Japan
2023 South Africa
2027 Russia or Italy or USA/Canada
2031 Argentina
That being said, before Argentina (and probably Canada or the US) can host a WC, they have to professonalize their structure (not players necessarilly, but the union's management). My point is that I don't think they'll give Russia a WC only 8 years after hosting one in Japan.