Greninja isn't any more of a promotional pick for X/Y than Cloud was for FFVIIr. Both games were either not even close to out or already out beforehand. Greninja was literally picked because Sakurai liked its design and was able to make it dance in his head right away. Otherwise, he wanted a Pokemon from X/Y because it's Pokemon, one of the biggest franchises in the world. Like Greninja, Incineroar pretty much got in because Sakurai wanted a Pokemon from the latest game, but both Smash games were still out after the actual Pokemon games released, making them pretty lacking in the idea of adding to advertise an upcoming game. Never mind that it had nothing to do with why he saved a spot for Pokemon. Cause again, it's Pokemon. That's all the reasoning that's needed. They were also added significantly before the Pokemon games were coming out, more in-line of "they were relevant timing of when to add that Pokemon".
Only Roy and Corrin were actually chosen with advertising as part of their reasoning. They needed a clone, and Roy was picked to also help promote his game. If it weren't for the clone factor, he wouldn't have been in. Corrin was from an upcoming game, something being looked at, but if it weren't for the fact they could be unique, they wouldn't have gotten in. There is no such thing as pure advertising picks in Smash either. There's always more context than that, which Sakurai literally mentioned the full story, to make sure people realize that.
Keep in mind it is possible that they chose Hero as Luminary being the main face for advertising reasons. But we have zero evidence of that. It was closer to "the most relevant hero to use as a face" instead. Which, like every 3rd party so far, had some kind of relevancy in their franchise around the time they were added. It's not just games. Franchises are often multi-media. For all we know, a big part of why B&K got in is so they can have a game announced soon to coincide with the release. Who knows. Obviously, as said before, any character added is effectively promoting their franchise to some degree, but that's just a bonus, not an inherent reason.