We’ve known for quite some time about the troubled development
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The above image is a storyboard for an early version of the special drawn by Andrew Wildman, who has been selling his boards from his time on Doctor Who at comic conventions alongside his other artwork. But when Bleeding Cool’s Rich Johnston purchased a set, he discovered an early look at what might have been had Eccleston committed to the project.
One of the initial scripts planned for the episode featured the 9th, 10th, and 11th incarnations of The Doctor in a story ultimately similar to the one we saw play out in the special: Zygons travelling back to Elizabethan England and duplicating Queen Elizabeth, leading to all three Doctors meeting when a rift in time and space opens up and dumps them all together. Wildman’s storyboards depict the Ninth Doctor seeing said rift opening, and finding himself in a forest looking straight at the 10th Doctor (and two Queen Elizabeths).
While we know that Eccleston would eventually decline to return, prompting some hasty re-writes (and even a version that just starred Jenna Coleman as Clara) by Steven Moffat. Still, Wildman’s boards offer a unique look at what might have been: the true meeting of the then-most-recent versions of The Doctor that many fans desperately wanted for the 50th Anniversary. You can see a few more of Wildman’s storyboards at the link below.