This is inspired by the currently ongoing
revamp of the NeoLemmix Introduction Pack, but is something that pack creators might find useful in general.
Allow creators to pair levels with image files, which get displayed where the pre-level text normally would be. (The image would take up less than the full screen, so as to leave a border of the preview screen's dirt texture.)
For the intro pack in particular, this would allow us to
show the new feature the level is teaching, avoiding the situation where the new player can't work out which object is, for example, the unlock button. Anything we want to say about the UI, such as the number that indicates how many lemmings remain in the hatch, could be shown much more clearly with a partial screenshot and an arrow pointing to the feature.
Also, the Lemmings font (don't know if it has a name) is nice and comfortable for level preview screens, where the amount of text is limited to the level name and single words, but dreadful when trying to read whole paragraphs. We'd be putting new players off before they've even had a chance to start the first level. The ability to make our own pre-level images (which can contain text) would allow the player to read about the game mechanics much more comfortably, and when necessary, we could fit a lot more text on one screen than we can now.