As has been said by others, for Australia I don't see promotion/relegation being viable in Australia in the foreseeable future. Firstly, the clubs looking at potential relegation are, compared the current NRL clubs , small both in terns of support but more so in terms of finance. Even allowing for the money they would get with joining the NRL, would anyone seriously expect say Mt Pritchard being able to compete for the corporate dollars against the likes of Parramatta, Souths or the Broncos? It would be a complete mismatch.
Also add in, could you imagine what would happen if say teams like Newcastle, the Cowboys, Warriors and/or Broncos got relegated for teams like Mt Pritchard or Wentworthville? We already have people saying the NRL is too Sydney centric yet you would open the door, potentially, to having more Sydney based clubs in the NRL at the expense of regional or non-NSW based teams.
Then add in the infrastructure requirements. Where would these new teams find the 20,000+ seater stadiums to play at? Few, if any, would have stadiums which meet current minimum NRL requirements, and if the new teams were in Sydney, you can't have say 10 teams playing out of 2 venues in the same weekend.
I'm not saying we don't need a second division competition. Merely that I don't think promotion relegation would work in rugby league in Australia.
Also using the EPL as a guide, how common is it for teams who get promoted one year to wind up being relegated rather quickly back to the 2nd division? Even then if some stay up, they more often than not hover near the bottom of the ladder.