[@Nythra Kthunae] To clarify, Shepard not existing wouldn't affect the Blitz all that much. If you don't choose War Hero for him it's as if he didn't contribute to it at all (despite being present). So think of it that way. 

We're only going to interact with canon characters that are entirely necessary (IE:Saren, Benezia, etc.) There aren't really any secrets at the moment despite what Dabomb said, we're really just using the Mass Effect Cannon as a plot frame.