That would be great. Although, if he's going to do any teasing, it might be better to start tomorrow, since Friday is the release of Tropical Freeze in America.
As I recall, the POTD prior to Little Mac's reveal day was Toon Link swatting DK with the HRB, and Sakurai subtly hinted at something more to the picture. If we don't get any DK-themed tease tonight, then we most definitely could tomorrow.
As for the Phantom, that was part of Zelda's photo batch on the official Smash site following her reveal (IIRC); these don't come with captions. And of course, it was the lack of captions that encouraged heated speculation. If Sakurai is to give POTD spotlight to the Phantom (and Zelda), then I'd venture that it would come with some more clarification, possibly even an outright explanation as to what the Phantom actually is.