Interesting characters get little to no actual development in the movies
It would have been nice to have seen more of Tom Riddle’s backstory, particularly when it came to Marvolo, Merope and Morfin Gaunt, the pure-blood family he’s half-descended from. Their inclusion could have given much needed depth to Voldemort’s character, but instead we got stuck with some weirdly implied Bellatrix affair. Come to think of it, those dark wizard house elves could have done with a little more screen time, too.