I’m reasonably confident the reason the finale left these posts vacant is that the show was out of significant enough characters to justify the screentime.
Varys was Master of Whisperers, and Tyrion will be hard-pressed to find somebody who matches the wiles of The Spider.
Grey Worm was meant to be the new Master of War, but he left for Naath with the rest of the Unsullied after Bran was named king.
Who will Tyrion and Bran find to fill these posts?