This idea's probably not fleshed out enough to implement, but I find that I need to mark or leave text comments on certain keys or points in the timeline. Some animation techniques require a lot of keys to be bunched up together or in odd (not immediately "readable" arrangements) and it gets hard to modify them when it comes to it.

It'd be nice if I could give a key a little marker or a custom mouseover tip.

I could kinda hackishly use a comments Event to do this but the event timeline is always on the very bottom of the dopesheet. Though, even then, it would only work as a marker for a point in time and not a marker for a key.
(Actually, the draw order timeline is kinda inconvenient to reach at the bottom too. Can't imagine where or how it should act to make either one more reachable though.)

@Shiu Any thoughts on this?

One example is when I have an oscillation. I'd want to mark the upstrokes or downstrokes to tell them apart immediately and be sure that I'm duplicating or modifying what I'm suppposed to be, maybe easily adjust them as a group (with the fancy new Adjust tool. ; ) ).

Another is in a standard walk or run, where I'd mark the point where the foot is planted, and when the foot leaves the ground again. The timeline gets a bit messy when I have to stabilize the foot on the floor with a combination of a few extra keys and some custom curves so it doesn't smoosh into the ground or slide around.

I'd also mark the ideal entry point of the walk/run loop in the timeline— this is normally somewhere in the middle, closer to the end, when constructing and animating it in the classical way. Animating it with the entry point at 0 is really messy in comparison. (The entry point is something that's implemented on the code side. Just a state.time = entrypoint right after state.SetAnimation. I'm not asking for that to be natively supported by Spine animation data in the json though. In other entry point cases, it's something that also gets adjusted based on game balance and not something the animator should have to do.)

In this case too, there are bouncy things that lag behind the actual foot plant, like a sword in a scabbard that bounces about 1 or 2 frames after the foot plant. Meanwhile, these keys are surrounded by other keys that define other little things.

For a lot of key pose-based animations with swaps, the slot image rows get a bit difficult to read and messy to move around. Actually, this is probably a separate feature that'd be handy: If you mouse over a slot image key in the dopesheet, it'd be cool if a mouseover tip would show what that key was doing: by showing the name of the attachment it keys, or showing the image, or null, or I dunno. It'd probably also be nice if a slot image HIDE had a different color or symbol (for the same reasons).

There are a few other cases I can't recall off the top of my head. It does come up though.

Yeah. Labeling a timeline position is definitely different from adding a comment on a key.

By "marking" keys, this is what I meant: I figured the rectangle representation of a key would show a really small colored square or some other distinction on it if it has a comment (and so the user knows that you can hover over it and there's something to see). I kinda thought that would help make some meaningful keys stand out in an otherwise homogeneous set of keys.

Of course, if you started leaving comments on every single key, this would stop being useful. xD But you could at least still mouse over them and tell what they're for if you've commented them meaningfully.

Ah, sorry. Clarification: I meant when you mouse over, the name of the image being keyed into would be visible. The name on the left is the name of the slot, not the name/path of the image (nor would it make sense if it were the name of the image since the image of the slot changes).

What would be useful is to see the image corresponding to the value bound to the "name" field of the key in the json when you hover over that key in the dopesheet. I wasn't suggesting the mouseover would show the name of the slot. :p