We're gonna need a plot, characters, weapons, aliens, gameplay mechanics! Luckily I have my trusted team of b.net writers and game designers to help me out.
Will update the OP with good ideas.
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The Force/Psionics/Biotics/Whatever- needs to be in the game if they want the franchise to reach Star Wars levels period. RPG elements- social players love seeing their character get stronger. Third person perspective - again, social players love seeing their character get progressively more cool looking. Also the camera angle is just better. Lots and lots of guns - keeps the game fresh when you can play with a variety of different weapons. Multitude of distinct enemy factions- again, keeps the game fresh for longer. The gameplay of Mass Effect 3 MP (the golden triangle perfected, guns, melee, and powers all mix together to create a sublime experience) combined with an open world full of quests that you can explore alone or with friends from the Borderlands franchise combined with a persistent shared hub similar to Guild Wars that would allow you to interact with the global community to form parties or trade = could be amazing. I'm not really sure what they should do with PVP, but if they want to have a PVE campaign that really and truly is legendary (and isn't just an overpriced movie), that's what they should aim for.