Val Kilmer Was Not Originally Written Out of the New Willow Series

According to Entertainment Weekly, Val Kilmer wasn't written out of the new series Willow until right before production started.

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Showrunner Jonathan Kasdan said "it wouldn't be Willow without Madmartigan" which is how he felt about the hero played by Val Kilmer in the original 1988 film.
Madmartigan still remains an important presence in Kasdan's new Willow series on Disney+, but the idea wasn't always for the character to remain off screen.
"We really wanted Val to come be in the show," Kasdan tells EW. "And Val really wanted to come out and be in the show. I remember going to see Val right after this thing started to get some momentum, and I said, 'Listen, we're doing this. And the whole world wants Madmartigan back.' And he was like, 'Not as much as I do.'"
Kilmer has struggled with his health after a battle with throat cancer.
He reprised his role as Iceman in this year's Top Gun: Maverick, but it was only a brief scene where he typed most of his dialogue. "He embraced me when I left," Kasdan remembers. "He picked me up, and he said, 'See? I'm still super strong.' And I was like, 'Great.'"
Kasdan was ready to bring Madmartigan back. "We started building out the first season with the intent of having him appear," he says. "[It wasn't clear we couldn't get him] until pretty late in the process, frankly."
As COVID-19 forced the world into lockdown, many companies had to rethink their approach to production. "As COVID overtook the world, it became insurmountable," he explains. "We were prepping in the spring of the year that it was most happening. And Val reluctantly didn't feel he could come out. We had to figure out a way to preserve the story we wanted to tell with him about how his story was playing out."
This does not rule out the possibility of Madmartigan appearing on the show in the future. Today Willow launched its first season, and Kasdan is eager for a second and would love to get Kilmer on set now that COVID protocols have been eased.
"We wanted to leave open the door to any possibility in the future and also honor the spirit of him," Kasdan says. "We've tried to do that and work with him in a way so that he is felt and heard, if not seen."