So my new wish for EU is music.
My FAVORITE element of IoA is the music, and I hope they will do similar with a soundtrack specifically composed for EU. Now Nintendo, Potter, (and I assume) HTTYD have existing music for their IPs, but the hub area, hotel, and Classic Monsters should be great opportunities for unique music that will help create their atmospheres.
On top of that, I'd love to finally have a Universal attraction with its own unique theme song. So many Disney attractions have theme music created just for the attraction that transcends the ride experience and is immediately evocative of the feeling the ride creates. Grim Grinning Ghosts, Great Big Beautiful Tomorrow, Yo Ho (Pirates Life For Me), It's a Small World, just to name a few, but even some non-Disney attractions have successful music tracks that have become beloved. I always think of the music from Monster Mansion at Six Flags Over Georgia as a great example of music elevating the ride experience beyond what you expect from a Six Flags.
I think the ideal candidate for a ride theme song will be the Classic Monsters dark ride. There should be something unique that fits the theme and you walk out of the attraction humming. A real ear-worm that sticks with you!