With Hero's "demo" coming up tomorrow, I would advise to keep a sharp eye on the entirety of the video. With Nintendo giving the heads-up that ver. 4.0 is coming as well, and the possibility that Sakurai might delve into the additional content within that version update, be on the lookout for potential "hints" to the last two DLC fighters. During the ver. 3.0 "direct" where they showed off Stage Builder, there were a few custom stages that had a dragon theme, and then there was the witch-themed stage, which IMMEDIATELY (at least that was my case, and certainly many other Banjoroos as well) made me think of Gruntilda; thus Banjo & Kazooie potentially being hinted. Of course such details at the time were mostly discarded as possible hints, but E3 2019 in a way validated said hints (especially the witch-themed stage). Having said this, just be sure to keep an eye out for any "curious" potential hints for the remaining two DLC fighters, and/or future content.