In my opinion an 18 team competition is manageable without divisions and conferences but a 20 team competition would benefit from a further breakdown. You need only look at the 1995 ladder progression to see why.
I don't think you would need two conferences. The use of derby performances within geographical divisions determining home finals would be enough.
If for example the next four teams that are admitted to the comp are from Central Coast, Perth, New Zealand and Queensland, then I would suggest the following:
Queensland Division (Brisbane, Gold Coast, North Queensland, QLD4)
Eastern Sydney Division (Cronulla, Manly, Souths, Roosters)
Western Sydney Division (Canterbury, Parramatta, Penrith, Wests)
Southern Division (Melbourne, Warriors, NZ2, WA)
Regional NSW Division (Canberra, Central Coast, Newcastle, St George Illawarra)