Surprisingly, as of right now, I do consider it a possibility that the secret boss is an "Eraqus" Nobody. But not in the way you're thinking. Xehanort's experiments are what made the Nobodies, so a Nobody couldn't have resulted from either MX or Eraqus releasing their hearts. However, we have to consider the special predicament of Xehanort the apprentice. Three hearts. Look at Sora. He too had three hearts. But Sora produced two Nobodies, one from the fusion between his and Ven's hearts, and the other from Kairi's heart. Wouldn't it make sense that two Nobodies were made in Xehanort's case as well?
Xemnas is the Nobody left behind by the fusion of Terra and MX's hearts, there's no doubt in my mind about that. After all, both he and Xehanort's Heartless are confirmed to have both traits of MX and Terra. Surely Xehanort's Heartless was [i]at least[/i] comprised of MX's heart. And then, of course, Nomura hinted recently that Riku may have Terra's heart within him:
Which would mean, naturally, that Terra's heart was a part of Xehanort's Heartless.
So if Xehanort's Heartless was made from Terra and MX's hearts, then Eraqus' heart was separated. That makes sense considering that Terra seemed more concerned about protecting Eraqus' heart than his own, so I believe he prevented it from being bound to MX's darkness like his own.
This begs the question of what happened to Eraqus' heart. And I think it's a non-factor at the moment. He had hardly any darkness in him, so he would, I imagine, create a weak Heartless anyway. It may have been turned into a weak Heartless, or gone somewhere else.
The important part here is that, when we compare this to Sora, then Eraqus' heart being released should produce a Nobody. He would be the Namine-type Nobody, the one not created from a vessel (which is unfortunately still a plot hole, but perhaps can be better explained if this theory is true).
Nomura has opened this up for possibility:
But what about the time issue?
That's obviously the biggest problem.
When I was considering the possibilities of who the secret boss could be, time nearly [i]always[/i] seemed to be an issue, no matter what I came up with.
Master Xehanort created the Organization outfits and the blue sabers are obviously a hint at a connection to Xemnas, so I am convinced that the secret boss is at least connected to Xehanort in some way, just like the two previous secret bosses (the Enigmatic Man and Lingering Sentiment). But how? As was pointed out, it appears that the secret boss occurs after the destruction of LoD (as you'll note by the sky). Meaning that this fight should probably occur around the time Aqua is bringing Ven to LoD. That's after MX is gone. But he couldn't have produced a Nobody, as I said. And the only thing that could put Aqua into contact with Xehanort doesn't happen until the [i]future[/i], until well after she went into the Dark Realm.
So, basically, yeah. Time travel. I can only hope that, if this is the case, it'll be non-canon. Or the time traveling plot device will be "irrelevant" at least...
After that long winded theory I do have a much more to-the-point alternative. One that I find a tad more appealing in some respects.
The secret boss is MX's Lingering Sentiment. Not sure if this has been mentioned, probably has. But, I mean, Terra had one. Vanitas had one. Aqua [i]might[/i] have had one. Not sure what the qualifications are at this point but hey, we've got human Xehanort, Heartless Xehanort, and Nobody Xehanort, naturally there'd be an Unversed Xehanort (considering that I believe LS's are to Unversed as Organization XIII are to regular Nobodies, just more powerful essentially). And this happens at pretty much the perfect time- directly after Xehanort released his time. It's the best window for a non-time traveling secret boss, at least.
The LS seem to utilize important objects left behind by the complete being (Terra's armor, Vanitas' suit), so in this case, MX's LS is using his cloak, which we saw at the beginning of the game.