Thought this was thread-worthy as he said before he would never return for TV.
https://www.radiotimes.com/tv/sci-fi/christopher-eccleston-doctor-who-return-newsupdate/
I've never been a fan of multi-Doctor stories. When I worked on the series, I had really strong ideas about what works and what doesn't, and I always think that multi-Doctor stories are a bit of a cash-in, and a bit of exploitation.
"Creatively, they never worked for me. I looked at the script for the 50th anniversary and I felt as soon as I said I wasn't doing it it got better because, well, if I'm not in it, it's better. The creation of the War Doctor introduced a whole new facet to the canon."
Interestingly though, a later comment from Eccleston suggested he would consider returning to Doctor Who in live-action for a solo storyline following the Ninth Doctor.
He added: "The Ninth Doctor, in particular, is a one-man band. Definitely. So he doesn't work with other Doctors. If you want me back, you'd get me on my own."
https://www.radiotimes.com/tv/sci-fi/christopher-eccleston-doctor-who-return-newsupdate/