I am pretty sure Ryu is just a placeholder, in all honesty, and that those two posts are meant for another fighter at some point in the week. The posts can be edited and replaced by another character when Nintendo decides to do so, anyways.
I don't think anything is on the way. Here is my theory about what that tweet really means:
Nintendo announced Lucas and had a release date planned for him. They had Roy and Ryu ready to go, but never even slightly hinted at this. Much to Sakurai's dismay, however, the patches including their data were data mined and the characters were leaked, moves and all. Right after the leak, Nintendo surprised us by announcing both Roy and Ryu alongside the Lucas released and then releasing them the same day. Since then we've had no specific characters to look forward to.
What happened? If they hadn't already been leaked, it would have made no sense for Sakurai to dump two unannounced characters on us the same day as Lucas. That is a huge huge waste of hype potential. And then after giving us a constant string of characters to look forward to, one at a time, he just gives us three in one day and then nothing to look forward to for months and months. An update with new costumes and stages comes, and no character with them.
I suspect that Sakurai's original plan was to release Lucas and then announce Roy the same day, to be released with the tourney mode patch in August. When that patch hit, Sakurai would surprise us with an even bigger announcement - Ryu coming to Smash Bros! Ryu's original release date was probably intended to be fairly soon. It's possible that something would have been announced when Ryu was released, but with the ballot closing just a month later, I think Sakurai would have just left us with all of that to chew on until October 3.
Of course when the two characters were leaked, this whole plan was totally ruined. There's no point in holding back the characters once everyone already knows they're on the way and have even figured out how to hack the game to play as them. Rather than pointlessly and annoyingly parcelling them out over the coming months, Sakurai just released them all with Lucas, and his carefully planned hype train came to a crashing halt.
This tweet is probably just a relic of that original plan. Nintendo's twitter is probably automated in some way, and had a series of tweets scheduled for different times over the summer corresponding to the announcement and release of Roy and Ryu. Most of these would have been deleted, but evidently they missed one.
That's just a theory, but I think it's the best explanation of this accidental tweet. Sorry guys, but I wouldn't get your hopes up.