Maybe the NRL should look at Sydney clubs sharing all gates, or maybe divide the stadiums in half and allocate one half away seating and the away club takes all or the majority of the gate for that section. The NRL could offer financial bonuses to clubs involved in games that draw particular target attendances and fine clubs that fail to meet minimum targets. Might provide a bit more motivation for clubs to promote games, regardless of it not being their home game.
Could also introduce reciprocal memberships for all Sydney based clashes.
It is really at the point where a collective solution is needed in Sydney that all clubs buy into, particularly if stadium rationalization is going to work. A lot of the recent pushes around membership have been aimed at competition between clubs rather than an inclusive approach that benefits everyone.