Maybe ad in a line to Palpatine’s dialog to Grievous that implies that he is planning to get rid of him.
Sth like:
“Your loyalty won’t be forgotten general”

Although I think that Grievous fate isn’t really important, but I would love to imply that Vader killed him.
This could be done with a close up of Vader’s fist, while hearing Grievous choking followed by the sound of metal hitting the floor. This little sequence could be squeezed in between the hologram scene and Vader’s first appearance on the star destroyer and you could ad Palpatine’s “you have done well Lord Vader” while they are supervising the creation of the Death Star, to finally conclude the chapter of Grievous.

While we’re at it why not include a reference to the clone wars episode about the giant kyber-crystal on Utapau to the hologram scene?

Anyways back to the question, I really think that Grievous fate is really that important and could well be left ambiguous.