Hermione could be an interesting choice. She's already very intelligent and powerful, and if she were a shadow wolf Animagus, she could use her new form to access restricted areas of Hogwarts at night for research. Also, her wolf form could help her in battles, using the element of surprise.
Neville could be a great main character as a shadow wolf Animagus. He has always been underestimated, but as a shadow wolf, he could gain confidence. His wolf form could help him protect the plants in the Hogwarts gardens better, and he could use his new abilities to stand up to bullies like the Slytherins. He might also discover that his family has a hidden connection to Animagi, which would add depth to his character.