Originally additional levels, but after playing through other submissions I most regret not adding music. This was the first game jam where I got something out that wasn't completely broken and unplayable and the first one where I also had time to look at what other people made, and I hadn't realized before how common it was to use free music assets for jam entries. I'll probably do the same next time. I did play around with an online synth for a while a couple days before the submission, but didn't have time to commit and make something decent with all the missing features and bugs that needed addressing.
This is an important one that always gets mentioned when you leave it out. I spent hours finding and downloading music off of itch and categorising it for games. Many musicians make this easy by adding tags like horror, fantasy, sci-fi, etc. to their songs. Now, I can just get a playlist together quickly.
Also, don't forget sound effects. They're just as if not more important than music.
That's a great idea! Sound effects I've already tried to add when possible, but I do put off working on audio in general mainly since it's by far my least experienced game development field (music in particular), so I never know how much work it will actually be and am afraid it'll take up too much time. I should probably build up a library like that too.