All the people pointing out that there's been instances of people theorizing roster number over missing spaces before are missing why a lot of folks are latching on to this in particular.
There's an extremely weird, extremely specific pattern that box has, a pattern that, for all intents and purposes, basically confirms that one of Palutena, Pac Man, Robin, Shulk, Bowser Jr, Duck Hunt, Ryu and Cloud have an echo. Counting what we know, and that echo (which is extremely likely Ken), the other side basically completely and perfectly lines up with the pattern the box is showing. Way way too fittingly to not at least consider the possibility. That leaves us with one spot missing on the side we can see. I'm 100% convinced Ken is the only echo left now, at least echoes for chars we currently know, and that no other echo exists because it doesn't fit with this pattern at all. Taking this into account, if we really only have one echo left, is it really weird there might only be one newcomer left as well?
Now of course the box might not be showing everyone, or that it's a placeholder (I doubt this one, the design and the way the chars are organized is way too deliberate for it to not be final), or whatever. But the possibility is there to consider. In any case, I'll personally take this as Ken confirmation on top of being the only echo left, and basically expecting Incineroar as the only newcomer to be revealed. I'd be pleasantly surprised otherwise, of course!