Only commenting for the parks I've been to recently, and have well-formed opinions on.
Movie World - Dark Ride replacing Justice League. Thinking along the lines of Duel/Curse at Alton Manor @ Alton Towers, use the same inherent ride system but build a better theme around it & remove the shooting mechanic altogether. Possible theme I had in mind, though a super unrealistic pipe-dream, is that it's a continuation of the OG JL theme, with inspiration from the Suicide Squad movie/video game, where the storyline keeps the 'Metropolis invasion' aspect (maybe even still by Starro, but the actual Starro, not that weird humanoid the ride chose to represent it), and the Justice League are the cause instead of the fix. No working with them to save the city, no laser blasters to hit Starro eyes, you're just a mere spectator, watching as the heroes you previously helped save the same city are now mindlessly laying waste to it.
Dreamworld - Modern Water Ride either extended out near ST, or attached to the Ocean Parade expansion. Much as I'd like to say a full on, balls-to-the-wall DCR competitor, and disregarding the obvious reasons why DW making a water ride might be ridiculed by the public, they need one of some form in their dry park, especially now as WhiteWater World is seasonal. Diving into pipe-dream territory again, a modern Intamin log flume like the one going to SeaWorld San Antonio. While being launched like ST, it's clearly unique in comparison to any water ride in the country, and lends itself to much more detailed theming than the other option, a PowerSplash (which'd also inevitably be compared to Storm & probably Steel Taipan too).
Sea World - Possible Hot Take: They don't need new rides, they need new exhibits. As much as a dark ride there'd be cool (even more so after seeing Levi's station), other than one of them they've got all the rides they need. Therefore, in my opinion, demolish VR's building completely, and replace it with a dedicated indoor aquarium facility. The Gold Coast is getting well themed rides aplenty, their sister park already has a better water ride than SW could ever get, but the closest (in of itself lacklustre) representation of an indoor aquarium is 2+ hours away. And, if they're truly worried about SeaWorld US's animal cruelty concerns, we have fish tanks in our own living rooms, if these tanks are better designed & stock more interesting species than those, there'd be nothing for the public to balk at.
Aussie World - I've got two, dependent on the route they want to go as a park. If they wish to stay a completely regional & family oriented fun park, a simulation ride of some form might do them well, seeing as they previously had one but it's been SBNO as of recent. But, honestly, they're filling that role pretty well right now, and there's no real glaring gap in their family lineup that I can see. However, if they wish to expand & attract less local visitors that skew older, they need a marquee coaster. Using FunSpot as a reference, adding a coaster that's somewhat unique to the area while not being super expensive & insanely thrilling, just something in that middle ground of accessible thrill at an acceptable cost, would do wonders. If RMC were established in Australia, a newer Raptor clone'd be a home run for this. Seeing as they aren't, however, something along the lines of a new-gen Vekoma STC/Top Gun (or really any small-scale Vekoma Thrill Coaster) or a compact Intamin LSM Coaster could fit that criteria well.