The notion is possible, but it's not entirely subjective. Objectively, Sakurai has included the main cast from the series that see playable representation, bar clones. Rosalina is the first exception to this, and isn't exactly comparable to Bandana Dee. Now, this could change, but I'm not dismissing something because it doesn't align with my perspective, I'm dismissing the claims of "likelihood" because it doesn't align with precedent, and with Sakurai's prior perspective and prior methods of inclusion. And again, it could change, but it's changed for a series like Mario, Sakurai's opinion changed for a series like AC; Bandana Dee is nowhere near the levels of these two exceptions.
The only similarity between Shulk and this situation is... you don't find Shulk likely, while I don't find Bandana Dee likely, as if that is the essence of the debate. Shulk and Bandana Dee are hardly in the same position, one is a relatively minor supporting character in an already adequately represented series (as far as its primary characters go), the other is the protagonist to his own series with some level of popularity and impact. If anything it's pretty fallacious to imply this makes their situations comparable.