I don't think we should have a five nations with a Pacific Islands team, because all countries have a proud heritage and diminishing those three nations as individuals could well cause conflict. However, possible tours would be satisfactory in my opinion.
With the expansion of the Tri Nations to Four Nations next year I for one will be interested to see how Argentina progress. However past that there is room for more expansion. I believe you can split this competition up into two divisions. Division One will obviously be the Four Nations of New Zealand, South Africa, Argentina and Australia. Then the second division will be the Pacific Nations Cup teams of Japan, Fiji, Samoa and Tonga. The format will be in the fashion that the Four Nations will take place: 6 games for each team, i.e. everyone plays each other twice home and away. At the end of it the lowest ranked team from Division One plays the highest ranked team from Division Two in a play-off for who is promoted and relegated. This can be done in a home and away leg tie with an aggregate score.
I feel this format will give the likes of Samoa the opportunity to stake their claim to play top flight rugby on a regular basis, and hence improve the standard of their rugby as a whole. The only problem would be enabling all of their players to be eligible for matches, which can be sorted out in conjuction with the Argentina have done so.
What do you guys think?