In addition to using the OPEN MATTE versions, maybe some few improvements from Doctor M’s The Enhanced ABC Cut could be used?

Now, a few more random ideas, though I have no idea how well this could work

Consistancy with the use of Polyjuice Potion. It should either always change the voice, or always keep the voice.
In Chamber of Secrets, the diary memories had a certain color tint to it. Maybe apply it to the Pensieve memory scenes in the other movies, for a consistant look?

Overall, making magic spells and effects consistant is something that can really improve the experience.