3. Mass Effect: Matriarch
A game set a few hundred years after ME3 which is a story of Liara, who you control. It would be cool to import an ME3 save file, which incorporates your decisions into Mass Effect: Matriarch so you can see the results long after the main trilogy's end. Things like, how are the races coping with their planets being nearly completely destroyed? Did the Krogan rebel again? What happened to the Rachni? What is life like if everyone is half synthetic? Seeing these story arcs fold out in a more complete way would be much more interesting to watch than the Extended Cut slideshow.
4. Mass Effect: Harvest
A game set on Earth during the events of Mass Effect 3. Play as a soldier fighting around the Earth, and it could tie in nicely with ME3, especially if you were under Anderson's command. The neat thing here is seeing events Anderson is talking about first hand, and only hearing hints of Shepard's journey. If this game was released fairly quickly, it may be able to generate War Assets for the original ME3 game, so maybe this one would be a downloadable game like Battlefield 1943.