I very much doubt it will be a TT-V6. Given the short timeframe for developing an engine package and getting it certified by Supercars for use, plus developing an actual car/chassis package, they'll likely carry over the existing V8, at least for now. They could use the 3.5L Ecoboost from the WEC Ford GTs, but it's likely far too expensive for use in Supercars racing.
I gather the fans are less than impressed with the sound of the turbo V6's vs the thundering 'Murican V8s. There's a reason Holden has dropped further development of the turbo V6s, and that's a lot of it right there. Even the Ford GT engine is not a very nice sound. Pretty awful sounding things, V6 turbos. Supercars is a pretty successful series, they risk becoming the new Nascar if they screw with a proven formula.