Since CVC looks to be mirroring the U.S. strategy around paid TV, not only I do expect Italy to stay in the Six Nations, but the tournament to expand to 7 or 8. South Africa is 100 percent going to join. I don't want to hear about "contracts" with the SANZAAR - you have clauses in contracts to leave if you pay enough. And I would expect a big push for Spain or Russia or some other large populace nation as well.
For context, USA paid sports TV has been completely flipped on its head over the past decade. With cord-cutting and streaming becoming the defacto winner, only live sports can bring consistent audiences. So the name of the game is to aggregate the largest populations to generate the highest advertising returns. The NFL just signed a 10yr $100 billion dollar contract with a host of networks including NBC, Amazon, ABC/Disney, CBS, FOX. I would expect CVC to do the same once they get the teams and ratings settled.
So in the future, a South Africa vs England Eight Nations match will be viewable in Ireland by Amazon Prime TV, Sky Sports in England, a paid provider in France, Wales, Italy, South Africa etc.