Basically, how about basing skills off of a name from something like a song? Basically, something like [Everything will freeze] (song from undead corporation), where you base a skill's power on the skill name that is limited to a song's name (something like time stop will work with the song i've chosen).
This is not a practical idea, but it might be fun maybe. Limiting your skills to only the song titles is a pretty creative way to test out your creativity.
You also have the option to use the lyrics as skill names too, to lighten the challenge. (I have no example for this, figure it out on your own)
Yeah honestly the thing is that this isn't much of a challenge considering that any word can be a title of a song, but idk i'm out of novel ideas.