Really it depends on if he knew his health was failing or if it came out of nowhere. Some authors, if they know their time is short and they might not finish their life's work, leave extensive notes… but the creative field is such that its extremely rare they'll have spent too much time actually scripting or writing the ending. (And often if they do, its an ending that was written far in advance, and is no longer really appropriate to how the story actually shaped up.)
Robert Jordan was pretty far into the final Wheel of Time book and had the last pages written, and most of the first 1/3... plus notes, so it was relatively doable to finish his story. Game of Thrones, Martin certainly has notes, but they might not be any more extensive than what the show had to work with.
Muira has assistants that he felt were ready to do more work to get chapters out faster, and editors that surely knew at least the gist, so it's feasible that it could be continued short term to reach some sort of ending, but it won't be quite right.
The Guin saga is still going on a decade after the author's death, but I don't know how the quality is there, and those are novels... slightly easier to transition. Hitchhiker's Guide got a single volume follow up to give things a slightly happier ending. It's not unprecedented.
Ideally, if they continue, they finish it in a completely different medium, like one more movie, so that its clear and distinct that it's a creative team rather than Muira... and the change in style is more acceptable.
It's just a shame all the animated Berserk stuff except the 97 version has been... not good.