Origin "works" because of the chip on the shoulder mentality of Queensland which goes back to the 1960s-1970s era when the QRL clubs were constantly raided by NSWRL clubs as they could not pay them as much due to poker machines being banned in Qld, which limited the QRL clubs revenue raising ability.
Whilst things have changed, its that intense dislike between the two states that makes it work.
The 1997 Super League Tri-Series was overall a flop. Its been quoted what players like Laurie Daley said, it just didn't feel like a NSW game. If you bring in other groups, be it nations (e.g. NZ) or other states (e.g. Victoria) you don't have that history to tap into and work with.
If you doubt me, look at the AFL State of Origin series, it started to die the moment they brought in the "Alliance" team (which was all the other states and territories outside of Vic and SA) as no one cared about that team at all. It only took a few years and the AFL State of Origin series was literally dead.
As to expanding the eligibility to include players from outside the current Australian rugby league pathways, you just wind up turning it into a glorified all stars game. For mine, that is what Test football is for, international teams.