Well the obvious answer is that it should be whichever team wins the previous Six Nations B (or European Nations Cup) championship. Last year that was Georgia. You could also make it more interesting by having the bottom team from the traditional Six Nations relegated to Six Nations B. But the fact of the matter is that all of the existing Six Nations sides (including Italy) are head-and-shoulders stronger than any of the B nations. There's not much point to an emerging nation like Georgia being absolutely crushed five times in the course of a tournament. Now that Argentina is in the Rugby Championship they are no longer a candidate for inclusion in the Six Nations.

As Jamie mentioned Argentina would be a good choice, but they are in The Rugby Championship now. With them no longer a possibility I think Georgia would be the best choice being the best of the second tier European teams.

There was discussion about an Argentina team based in Spain or Portugal as most of their players are in Europe anyway. Argentina are a good option as no other European team is in the same league as the 6 nations.

There are only 2 viable options: Georgia and Romania. Both these teams are leagues behind the 6 but with proper investment (resources, financing, structures) they could be of a much higher standard very quickly. Rugby is largely unknown in some Euro States, so I think the IRB should promote it more.

Romania