Since I don’t think there’s much question of whether people want this (yes) or how they want it to look (take the brush editor stroke preview and put it in the presets list), I’m just going to sum up the technical side of how it might be able to work, as I see it.
When we find or save a preset without a brushstroke preview, render the preview with KisPresetLivePreviewView (as in the Brush Editor) and save it as an image into the resource database somewhere. Unless is really is possible to get away with only generating them on-the-fly when they’re being looked at.
Whenever the brush changes, the new preview is temporarily cached, just like the temporary brush changes are.
Whenever Krita’s color theme changes, the previews must be regenerated. (Because non-color brushtips use the view text color as their color, and the stroke including the blending mode is composited onto the window background color.) Ideally we don’t update the previews of all presets every time the theme changes, because that would be not fun. It again would be better if we can only update them as they’re shown.
The brushstroke preview is added to the preset resource somehow so it can be shown in the presets list (since it is a generic KisResourceItemChooser, which only shows the data of resources). Logic and UI are made for showing the preview in the list (in KisPresetChooser).
But I’m not too familiar with how the resource system works, so this is all theoretical.