With as much lore they have to play around with, they should make a couple of prequels.
The From Ashes DLC made me really really want a prequel about the Prothean war against the Reapers.
Seriously, tons of stuff they can play around with. The Prothean's war with the Reapers, the discovery of the Rachni and the Rachni wars, the Krogan Rebellions, the Morning War, the First Contact War, and a big list of other crap. Or, even better, we can have a game following Cerberus's actions.
Hell, they could make some games on the side stories of the game. Garus's/Archangels squad during his war on Omega? Perfect example of something that could be expanded and made into a game. Or the Paragon Lost movie, that can be turned into a game as well.
They don't need a direct sequel to 3 yet, since they have so much interesting stuff in the current timeline that they could play with.