Personally, I wouldn't want the Shivans to remain a mystery forever. I get the whole mysterious uber enemy thing and all, but by the end, I want the questions about the Shivans answered. Otherwise, it's basically like asking somebody a question and then telling them that you will never let them know the answer. If you're going to ask me a question and I can never know the answer, I don't want to know the question in the first place.
Yes, I know the mystery behind the Shivans was one of the driving forces behind the story, but when the story ends (i.e. FS3), I feel that mystery should be resolved.