AO will never be open source. And that's not because Funcom wouldn't want it, but because they never could make it open source.
AO, like most games, uses a good amount of 3rd party libraries Funcom is paying licenses for. AO relies on these libraries to work and replacing them would take a considerable amount of time. For some of them there aren't even good enough open source alternatives available.
These libraries' licenses are not compatible with open source. Nor is Funcom permitted to distribute the source of these licenses.
Secondly there is the issue with how AO's toolchain works. AO's content is created using a series of tools specifically made for AO. These tools also were never designed to be used as stand-alone tools. They don't work outside of the office and especially don't work with multiple instances of these tools out there.
AO's tools assume and are build around a single development environment (their office network) and don't support the sort of remote collaboration and cherry picking of content/features you'd want of an open source project.
tl;dr AO can't be open source due to incompatible 3rd party licenses and incompatible toolchain