Players worldwide slammed the reported proposal, while a Pacific nations' lobby group said it was considering a motion to boycott the World Cup in Japan.
World Rugby (WR) said in a statement on Wednesday that its proposed 'Nations Championship', presented to union CEOs and players in September, would have a two-division format and include promotion-relegation for a "potential pathway for all unions".
"It is incumbent on World Rugby to champion and represent the whole game, not just the top of the game, and we are committed to working with our union and player representative colleagues to ensure an equitable solution that works for all," the statement said.
According to WR's outline, the tournament would start in 2022 and run twice every four years, excluding those when World Cups or British and Irish Lions tours were taking place.
It would be split into two conferences, with the Six Nations retained as one and an expanded Rugby Championship tournament, adding two 'Tier Two' teams to the existing four southern hemisphere nations, as the other.
Each team would play the other once, with the top two teams from each conference to play cross-conference semi-finals, followed by a final.
It would also provide qualification and seeding for future World Cups, WR said.
The body conceded that making changes to the international rugby calendar was always going to be difficult.
"With the proposed model incorporating competitions that are not owned or run by World Rugby, not all unions are presently in favour of immediate promotion and relegation," it said.
"We continue to consider the feedback, but remain absolutely committed to an eventual pathway for all."
WR said the proposal would be further discussed at a joint meeting with unions and player representatives, expected to be held later this month.
(Reporting by Ian Ransom in Melbourne; Editing by Nick Mulvenney)
