Why would your average punter go and watch and cheer on a rival club in a national league? I wouldn't, and I've been told I am bigger than average by my doctor.
Who's asking you to go cheer on a rival club?
Well in case you missed it, the conversation is about clubs playing in a national league and not a regional entity. I'm sure bums on seats will be some form of measurement of how successful it is, so if your rival club is the only team in the league, do they expect/want other people in the city to now support that team?
But your local club will be in the 'league'. They might not qualify for the finals stage (whatever form it takes) but you can either go watch a couple of matches as a neutral at that stage or enjoy a couple of rounds of golf.
One idea I've had is that instead of having the top club or two from down south advance to the finals stage, you could instead have Mainland/Southern United come together for it, if it was decided that would work better in those areas.