Double post for some speculation!
One user on German forum Freizeitparkcheck.de has pointed out this foundation which apparently is similar to the base of a flying theatre. Could it be a coincidence or is this going to be the second attraction in Rookburgh?
He suggested Vekoma's Panoramic Flight Simulator (However I imagine most Flying Theatre's have a similar base so it could be any manufacture).
https://www.vekoma.com/index.php/specialities-and-attractions/panoramic-flight-simulator
A Flying Theatre would suit the steampunk/flying theme of the area, and it would be a family attraction with a decent throughput (Vekoma's model has a minimum height requirement of 1m and a throughput of 900phh - which is higher than Raik's). Also when Maus au Chocolat opened the park creative team said in an interview they liked the how the technology meant they could refresh the attraction by changing/updating the film every so often - a Flying Theatre would also give them this freedom.
An argument against it would be that the park tends to go for more unique attractions however, like most PHL projects, they have probably been planning for many years by now and the decision may have been made before they knew Europa Park was building one.
Also I question what would the film be? Flying over real world landmarks doesn't really seem very Phantasialand to me - the park strives for immersion in other worlds.
This did remind me of the fact back in July the park had 2 helicopters flying/hovering overhead for a whole day seemingly filming the park. And then a couple of months later Chris Theis, executive assistant at the park, posted this picture taken from a helicopter over the park.
Maybe a flying theatre is coming?
Personally I'm not keen on the idea...