Clipping feature

[rotund apex]

Make a clipping feature that allow user to record their screen (you would need to specify the length of the clip before starting the recording)
And when you press a key it the save the last few seconds (depending on the length you previously set)

And if you have time you could add a feature that also save the player camera position rotation and fov changes over time in a .ma / .abc file (idk which one is the best for this type of use).

[limber surge]

already suggested

[fluid kindle]

Technically, lunar client already has this feature. At least in slightly older versions you have access to the replay mod. However, the whole thing is no longer supported, at least not for the completely new versions.

although that's not quite what you mean either