I love the voices that are currently available in the game!
In fact, I think it's great that they're all in the same general register and not wildly different.
That said, I think the next step would be to introduce a very low pitched, slow voice and a quicker, high pitched voice.
For the male voice, it might be a gravelly dwarf-ogre voice and a trilling halfling-elf voice.
For the female voice, maybe a surly, sinister voice and a higher pitched , cheerful, light-hearted voice.
After that, maybe consider moving on to alternate accents and more defined character personalities. That's probably "next level" for BG4.
