We're definitely still a few years out on any replacement for Arkham. There were a lot of internal talks (it reached pretty much every department's management levels so I'm going to hazard a guess they were pretty solid or final) of Arkhams replacement early-mid last year, so 2022 or beyond sounds reasonable.
During the announcement of Rivals, management made clear to staff that they had the mindset of moving the next few major additions away from DC branding, so hopefully that desire has remained.
Sea World expansion will be completed first, alongside a new slide for Wet 'n' Wild (assuming that's still going ahead) before anything substantial happens at Movie World again though.
I seriously hope they agree with Gazza and there's plans to move the Batman statue to Batwing. Have it positioned on top of the JL show stage overlooking Batwing or something.
Add this to the dream category though; what about a Wooden or Hybrid coaster that acts as a barrier between MW and the Studios. Would then also act as a border for that non-DC themed area that bridges the gaps between West, Scooby and Superman.