Wasn't it mentioned somewhere that the Secret Episode would be playable, and not just a cutscene? If so, I think it will have to do with
Aqua meeting Mickey in the realm of darkness. I mean, at the end there's not really any way to play as Terra and Ven, so you must play as Aqua. And
whether or not she met Mickey is pretty much the only mystery left regarding her.
If it is just a cutscene, then I think it will be Ansem's apprentices giving up their hearts to darkness.
EDIT: Also, Reian, Reconnect:Kingdom Hearts has not been confirmed as a future game. And since the title is backwards (subtitle before series title), I doubt it really is a new game. Also,
Re:coded's secret ending shows the exact same words.
Both times it is used is regarding Sora's upcoming journey, which Nomura vaguely says relates to KHIII. So I think Reconnect refers, not to a specific game, but to how all these individual stories games are "reconnecting" to Sora's main story, Kingdom Hearts (Nomura has said over and over again that the numbered KH's are Sora's stories). Thus each secret ending relating to Sora ends with Reconnect: Kingdom Hearts.