I’m happy with either of the first two, or North Brisbane as PR suggested, (especially if a third club southside is ever in the cards).
I don’t support any broad nomenclature which will not be serviced by said club. Personally, I would prefer the names Auckland and Townsville for the Warriors and Cowboys respectively. They may claim to represent the larger footprint, but they only play in one city within. These club names, however, don’t encroach on other club identities, although “New Zealand” should be reserved for the national representative side.
Sunshine State suggests they represent all of Queensland, which they will not, and therefore encroach on the identity of the 3 existing clubs. South Queensland, likewise, encompasses Brisbane Broncos and Gold Coast Titans.
The East Coast suggestion is the worst of all in this regard. Melbourne, Canberra and Warriors are the only teams not included in definition. (Yes, I have included all Sydney teams, even Penrith, they are close enough).