I don't see a Legendary edition happening. It's more complex than Mass Effect since 1, 2, and Inquisition share basically no assets between them, and 3 is on different engine entirely. Maybe a basic upscaling of Origins and 2 could happen, but EA hasn't really been doing those.
Does Dragon Age DLC ever go on sale? Will they announce a Dragon Age Legendary Edition after I buy all of it?
Also, ME Legendary supposedly only happened to get the reformed Mass Effect team up to speed, but I kind of doubt that since Bioware pretty much always has the bulk of their team on one project at a time. Plus remastering old assets on UE3 probably doesn't compare much to the work needed for ME4.