Scott Johnson says his preference is to design a comp that will be the best in the world, and that it would be hard to have the best comp in the world without NZ in it (
https://www.buzzsprout.com/843712/3934223-the-rugby-wrap-s-1-ep-11-with-scott-johnson)
So another option for a TT comp would be for NZ to increase the no. of their teams to 8, and include the Oz 5, plus a possible PI team, for 14 teams (
https://www.stuff.co.nz/sport/rugby/all-blacks/121523112/why-nz-rugby-could-now-rip-up-super-rugby).
This could make for a pretty even and attractive comp. It would make for a 13 week regular season before finals and/or a champions league, not including any byes, which is just about right.
If they were to have a champions league with SA/Arg or a finals series with the best teams from both conferences, it would allow SA to bring home their two from the Pro14, increasing their teams to 6, without depth being an issue any longer. Including the Jaguares would make for a 14 week double round robin regular season (including byes), which is also just about right.
This model would also allow them to keep open the option of including the top teams from Japan in a champions league or finals system, either now or in the near future. I think this will be one of their key priorities in designing a new model.