V8 Supercars won't change I don't think. They are getting close racing and great attendance at races with the Ford/Holden thing. Unless Toyota or LOL Mitsubishi can locally produce a rear-drive large family sedan with a V8, making sure the proposed V8 supercar version uses all the same components as the Fords/Holdens, and goes exactly the same speed, they won't let it compete. V8 supercars is a very tightly scripted formula these days...
They don't want any one make dominating the championship, or conversely, always losing. It keeps the crowds there. Plus neither Toyota or Mitsubishi (Chrysler is OUT, its not locally produced, which are the rules) have any sort of cred among people who like the championship at the moment.
At its simplest level, its an event and attendence driven operation, and if Toyota or Mitshubishi can't bring significantly greater attendance/fans, they won't be allowed to enter.
I would LOVE to see all different cars ala the Dick Johnson Ford Sierra era back in the late 80s but its not about true motor racing anymore, its almost to the point of scripting who wins, like wrestling. All those rule changes so Holden can't win as easily etc...