The point of such a feature would be for the user to be able to read the X-sheet and go “ah, yeah. it’s repeating.” or “it’s going back and forth.” or “it’s doing the stagger technique, in this specific sequence”.
Currently, I’m thinking along the lines of allowing optional text labels.
Like, if you can mark a drawing as a key, you should be able to label it with a few characters. “a1” or “1” or “A” or “AB”. Visually, the label can be right on thumbnail in the x-sheet UI. And any linked instances of the drawing can also have that label visible.
Those drawing labels could even be longer (like “mouth open”, or “blink extreme”) if there was a nice way to present it.
The benefit would be that not all drawings need to have the extra visual clutter. But if it actually helps you understand the x-sheet, then you can add it.
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Alternatively, I don’t know how you feel about the paradigm that in classic animation, every drawing has a number label anyway and that’s what they write down on an old-timey X-Sheet.
If the program generates those numbers/IDs on its own, there could just be an option to unfold/collapse an extra column in the UI that shows all the labels for the drawings on the layer you’re working on. The problem is that as you insert and remove drawings while pencil testing, auto-generated labels tend to change, or get jumbled up (numbers getting reordered, numbers skipping, numbers between numbers).
This second way is how Clip Studio Paint does it and has this little hitch, and it falls on the user to rename the drawings into a sensible order, or use a built-in function to rename all the drawings according to the sequence they appear in the animation. The up side is that the user has full control. It’s not really a tedious task, especially with the auto-rename feature.
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I’m leaning towards the first one, but maybe the second one might make more sense if it fits into some other features you have planned. I understand this requires some time for consideration though.