I don't see how creating a character and choosing their voice would ruin the game. Sure, the voice might not be a good fit like the past games that have no choice, but I think you'd forget about it pretty quickly. I've never played SR2, so I don't know how good their voice actors are, but GTA always does a good job.
In either case, giving us Create a Character after beating the missions wouldn't be a bad idea. We can already change clothes, although I find it to be annoying and takes way too long, you should be able to cycle through all the clothes in the store/closet without having to wait for the character to walk into the closet and get changed, which is why I never bother with the clothing.