After writing and directing the first two Guardians of the Galaxy, James Gunn has been fired by Disney for Guardians of the Galaxy Vol. 3 due to resurfacing of tweets of his that they deemed offensive.

Karen Gillan with her Guardians of the Galaxy co-stars has signed a letter in support of Gunn:
https://www.gamesradar.com/guardians-of-the-galaxy-cast-make-public-show-of-support-for-fired-director-james-gunn/

After it was announced that it has been put on hold, it is now been announced that production of Guardians of the Galaxy Vol.3, which would probably have seen Karen Gillan return as Nebula, has been delayed to February 2021.

Disney and Marvel Studios has reversed its decision on James Gunn and rehired him to direct Guardians of the Galaxy Vol 3.
Filming for Vol. 3 will commence once Gunn completes work for The Warner Bros. film The Suicide Squad which itself has been described as DC’s answer to Guardians of the Galaxy.

Guardians of the Galaxy Vol. 3 with my beloved Karen as Nebula will be released on May 5 2023.