I said this in the character thread that I support, but I'll say it here as well.
Whether you want to believe it or not. This is what I suggest.
Lower your expectations: Believe that we will only get Ken and 1 new character. Worst case scenario is that it happens and we've coped with the final roster months in advance and can start planning our DLC campaign. Best case scenario is that the box theory was wrong and we have way more to enjoy and look forward to as a surprise.
Keep an eye out for the music: Both on Brawl's Dojo and the entirety of Ultimate's website, there has NEVER been a repeating franchise that has music on the website. Every newcomer that came out after the website started has also come with music (and it's expected that Isabelle will have an Animal Crossing song next week because of the blog number switching due to the delays). This insists that the only new characters we can get are either from a new franchise, or from a franchise that does NOT have a song on the website yet. Amongst those, they include
(Mario Kart).
(Note: This doesn't include Bayonetta or Final Fantasy because the August direct showed us that they have no new songs)
Out of them, these have the highest possibility. Listed from most likely to least likely.
(Confirmed Isabelle, their song will most likely show up next week)
(Gen 7)
(Tails, Shadow)
(Elma, Rex)
Since it's rumored by Vergeben that we're getting a Gen 7... that's probably what it's going to be. But It still could be a Sonic or Xenoblade character.
Anyways, the point I'm trying to get at here is, watch out for the songs that get picked within these next few weeks. We'll be able to get a better idea on who will probably show up depending on what songs get put on the site before a character reveal.
Wait for the next direct announcement: If there really is only Ken and 1 echo left... theres only going to be one more smash direct for us to watch. If it doesn't happen in October, then it's most likely the case that this is all we're getting. If it DOES happen in October, we can allow ourselves to be highly skeptical of the box theory.
That's my advice for the situation we're in right now. Take it as you will.