The thing is though, Tangrowth was also there, so there are two Pokemon that are seemingly "out of place." Therefore, whatever the case, it's likely that they were both put in for the same reason... Most likely, both of them belong to whoever the new friend that Ash & Co make in this movie is. That makes infinitely more sense than it just being Tangrowth that's a random or whatever and Mightyena being this sort of cryptic hint of an evolution in a method of hinting GameFreak has never done before (as they have no connection to the actual merchandise. The only thing the merchandise has hinted towards is the characters in the anime getting new Pokemon, or merchandise of a new form or Pokemon being blocked out/shown as a silhouette).
Trying to pin that Mightyena as being a hint of a future evolution of it is really just looking way too much into it (not to mention that doesn't fix the thing about it being out of place due to it not being one of the actual protagonist's Pokemon. Meaning that for your speculation to add up anyway, it would have to belong to someone else who appears in the movie anyway, in which case it's much simpler and more likely for it to just be the Mightyena itself to belong to someone else in the movie. In other words, if you'd agree that either way that the Mightyena doesn't belong to Ash & co, then why not just stop there? Why must it mean anything more than just that? And why is the same standard not being applied to Tangrowth (we may not actually have a viable silhouette for it (...yet) but that doesn't mean that the same exact things couldn't be said about it)?). It's either a random, or belongs to one of the characters in the movie. That's it; it doesn't mean anything else, as that's not the way GameFreak hints at this kind of stuff.