theres very few times a stadium of 60 or 70k will be filled outside the games. perhaps AFL finals (if the lions are performing) and when we play England and India in the cricket - on average not even once a year.
The geography of the Gabba is also a massive limiting factor - theres major roads on 3 sides, with a heritage listed school on the other.
there was talk of the albion railyards. I'm not keen on this for a few reasons
1 - the gabba is being connected by cross river rail and will have a dedicated train station opening in 2025. adding another station would need more infrastructure to access it, adding even more to the cost. the gabba is already well served with buses and will be (by the time of the games) served by the Brisbane metro
2 - The gabba needs a rebuild anyway -- we almost lost the first test multiple times thanks to the state of the stadium. people complain it's 1 billion for 8000 seats. it's not just adding 8000 seats. its better media facilities. its better corporate facilities. better facilities for people going to matches.
i get the idea of a new stadium but from a financial and legacy perspective it makes no sense to build one