2. Protheans
Source of much controversy surrounding Mass Effect 3 to this day thanks to their race being the key to everything that happens in the entire trilogy, the downloadable Prothean character Javik was still an amazingly insightful and worthwhile companion if you forked over the cash. Essentially the linchpin for technological advance throughout the galaxy, the ancient Protheans created everything from the mass relays to the Citadel itself, essentially being the Nikola Tesla of the universe - leaving many unfinished plans and ideas that proved they were lightyears ahead of their time. Playing as these guys would be the perfect way to flesh out the world in the most immediately affecting way possible, as literally everything from world/planet design to certain weapons and the technology of the time alongside conversations about their own incoming Reaper invasion would make any Mass Effect fan weak at the knees.