having kerrigan stay more human would have been a cool option, but i don't think they would have had the time/resources to do it, as it would also impact legacy of the void.
it would be cool though, imagine a non-infested kerrigan still controlling the zerg, complete with a mecha-QoB wingclaw suit thingy.
i am sure raynor would be a lot more happy with that plot branch.
Why couldn't they just be more like nintendo and take extra time to ensure the game is going to be great? even if its some non canon side story type thing it would have been great to have that option as it would have expanded things by a lot.
Blizzard is known for taking their time until the game is great.