I find Erdrick an extremely peculiar character for them to include at E3. The Erdrick we all discuss, the Dragon Quest 3 Hero, is somebody who is a complete nobody outside of Japan, but within it nothing short of an icon who might not be Mario/Pikachu tier but isn't all that far away. Slime, which is much more popular outside Japan and immediately distinctive, and the Luminary, who comes from the most recent game that Square would want to promote, seem like they would make far more sense for inclusion at E3, where the primary audience is the west. Erdrick would be better suited to, say, a TGS reveal in Japan, where his primary audience is the one watching and the ones who would be blown away.
Erdrick not appearing at E3 isn't the end, but in fact him being a TGS makes more sense to me than him showing up there. That's where he'll be appreciated by everybody, not regarded with confusion by the public at large. Granted, Erdrick DOES have fans in the States and other locations, but relative to even other Smash fandoms, let alone the general public, which is who Nintendo wants to sell this pass to, they aren't that much. A September character reveal is entirely possible, if not quite likely, and I think a TGS reveal while framing it as a loose video for the western world would make more sense than trying to sell a character who's a virtual unknown to a mass audience while the people who WOULD care won't know for a while. Given, of course, there's no other specific reason that mandates that Erdrick must be the E3 reveal (Square demanded it or else they would pull the rights to Cloud, the reveal trailers for 3/4/5 all feature Erdrick, Sakurai just couldn't wait, or maybe something else).
This is written with the assumption that the leakers are completely infallible in regards to Erdrick. I don't believe in leakbait, at least in regards to Smash, but there's nothing saying that the guys who've been leaking Erdrick aren't somehow mistaken.
As an aside, I already feel bad for Erdrick and Banjo's fanbases. Assuming we get one reveal, then one of them is going to be treated as the "winner" who will be attacking the "loser" and their fanbase. If it's neither, hoo boy, those two will both be treated like crap for a long time.