When I asked if Earth would be the last multiplayer DLC he actually laughed at me and explained they are making way more money developing multiplayer content than they would single player content, though both are being made, according to the forum poster.
While the single-player DLC might be the Leviathan add-on, which is practically all but confirmed after files and voice actors have confirmed its story, the multiplayer add-on might bring a fourth enemy faction.
When I commented that I wish there were new opponents, he confirmed the studio was already working on a new faction, the poster added.
Whats more, besides the new enemy faction, BioWare is also trying to make cooperative missions, not just matches based on waves, according to the unnamed source.
He also says they have written plans on making 4 player cooperative missions that play like the single player, but he said that it was at a very early stage of development and cannot be guaranteed.