Not a bad theory, but there's one thing that I thought of that might keep Obsidian from this type of path. That is, the difficulty of playing DS then.
If your scenario is correct, then that means that Sion is our opportunity to join the Sith. Assuming we agree to join him, that means that playing as a DS character, we'd not only have to fight the Republic and any scattered Jedi, but also the Sith who are loyal to the other Sith Lords. In other words, we'd be fighting against BOTH sides. Whereas if we turn down Sion's offer, we'd only be fighting against the Sith, since we'd still be loyal to the Republic and the Jedi.
That's not to say that I wouldn't like this idea. I just think that Obsidian might want to steer clear of making the DS harder to play through than the LS.