Gold Coast is an obvious choice. We've already shown an athletics track fits in the ground (doesn't fit into the Gabba in current configuration) and temp seating brings it up to 40k as it was during the commonwealth games. Minimal spending and 45 mins south of QSAC, not too bad.
Then put $600M into Suncorp to bring it up to 70k with a new western stand and filling the corners in for opening and closing ceremonies plus soccer. Money will have to be spent on Suncorp anyway so may as well use this opportunity.
Put $400M into modernising the Gabba as a 35k stadium (sustainable size for the footprint the Gabba has to work with) which is more than enough for cricket and Fitzroy). Again, the Gabba is going to need money spent on it anyway so may as well use this opportunity.
Spend $500m on a new 30k rectangular venue at Perry Park, Ballymore, Showgrounds or Redcliffe for Rugby 7's and Olympic Soccer.
Brisbane Olympics then leaves a legacy for NRL, NRLW, AFL, Cricket, Soccer and Union.
Brisbane has a 70k Suncorp for events like NRL GF's, Magic Rd, Origin, Concerts, big NRL derbies, Wallabies, Socceroos, Matildas.
A modern, state of the art oval for cricket and fumbling.
A much needed mid sized rectangular venue for Reds, Roar, some Dolphins games, smaller Wallabies tests, NRLW Origins and Finals games and magic round overflow once we hit 20 teams.
1.5m spent to actually set the city's stadium infrastructure up for the next 30 years.