Personally, I don't see too much correlation between Ashley's promotion and the fate of Assist Trophies like Isabelle. Ashley has always stood as a potential unique inclusion, while characters such as Isabelle, Shadow, and Dark Samus have seen a topical surge in popularity because of how potentially easy it would be to shoehorn them in as Echo Fighters later in development. By the time we find out that Isabelle is an Echo Fighter of Villager, we'll probably already know Ashley's fate.
The only Assist Trophy whose fate could possibly be tied to Ashley's in some way is Kat & Ana. While both were present in Smash 4, neither have been revealed yet, and if Kat & Ana were to be revealed as an Assist Trophy while Ashley is still MIA, then it'll get people thinking, possibly setting fans up for a potential Ashley reveal down the line.
Right now, I'm just bracing for what might be posted on the Blog on August 3rd. It could be really promising, or really bad news for us ... or it could be the re-confirmation of something we already saw at E3, yet again. At this rate, it'll be the third one.