The current slayer helms should remain as the pinnacle of each tier.
Adding in a combined slayer helm of all 3 is just unnecessary and discourages end game gear diversity.
For instance, If a player was to ask what the best helm slot for slayer is, they would get 3 different answers depending on combat style. But if we had a grand slayer helmet, it would just be the same answer, making it look like we lack content.
I would much prefer to see a new slayer helmet added to the wilderness as it would be much more viable to add unique features.