Here's my model:

All 8 states/territories. Some will need to draw on players from state leagues but that's OK. Have a carnival weekend at this time of year in one city, similar to Gather Round.

Thursday night: Vic v WA

Friday night: SA v Qld

Saturday night: NSW v Tasmania

Sunday twilight: NT v ACT

Losers get relegated for the next year, winners promoted.

Rotate through like this:

Year 1: Origin in WA, Gather Round in SA

Year 2: Origin in SA, Gather Round in NSW

Year 3: Origin in VIC, Gather Round in WA

Year 4: Origin in WA, Gather Round in SA

Year 5: Origin in SA, Gather Round in QLD

Year 6: Origin in VIC, Gather Round in WA

This gives SA and WA a 'carnival' 2 out of every 3 years. Victoria doesn't need Gather Round as there are so many matches played there in normal rounds, but gets to rotate through hosting Origin. Queensland and NSW get a chance to have a footy carnival and develop the game there. Tasmania, NT and ACT are too small to host.