Okay, we all know Aizen's a genius because of his overly convoluted plan and his near omniscience, but speaking from a militaristic point-of-view, he's a retard. Harsh of me? No. Lets look at how he's systematically destroyed his own force before you start disagreeing.
First of all, Grimmjow defeated Ichigo and then got intrigued enough to kill him. Had Grimmjow done this, then Ichigo (someone Aizen notes has an incredible growth rate) would have been taken out right there and a lot of damage could have been avoided. He wouldn't be dealing with a Vizard, or rather, a Vizard with control of his power (more or less). Then Ichigo and his rag-tag team of would-be rescuers arrive in Las Noches, and instead of sending the top-tier Espada to kill them in one ferocious assault, he watches as the Privaron Espada get taken down one by one. The point where he should have gotten serious was when Aaroniero died, this should have been where he realizes his precious Espada are in danger. At that point in the story, it would have been best if Grimmjow had taken command of the Arrancar army because he had the right idea, "It's best to crush insects quickly once they've gotten inside right?". This was the best analogy for the situation. When insects enter a home, they often times drastically increase in numbers; Ichigo's team drastically increased in power.
Now Aizen stated that he kidnapped Orihime because he knew they would try to rescue her and Captains would have been lost in Hueco Mundo, but how would they know whether Ichigo and his team were dead? To be more clearly, had Aizen annihilated Orihime's rescuers on sight, the Captains would have still come, but this time there would be no one there to help them, and upon their arrival he decides to send more Espada to their deaths. What he should have done was attack them with the top 4 Espada and Kaname, Tosen and himself, that way, no casualties would have been on his side. Then instead of invading Karakura Town with 3 Espada and 10 Fraccion, he could be invading with 10 Espada and 11 Fraccion.
All of these mistakes and the seemingly quick deaths of his forces makes one wonder if Aizen truly is the final antagonist of Bleach. So far we know of 5 dimensions/planes of existence in Bleach; Soul Society, Hueco Mundo, Human World, Dangai and Hell.
The Dangai is simply an "in-between" world. It was a central world in "Memories of Nobody", but the wikia gives a vivid description of the movie so you can read up on it in that article, but I do not care to elaborate much on this world.
The first arc took place in the Human World, where we were introduced to this mysterious "Soul Society" that Rukia continually mentions, but I doubt anyone really put much merit on that. The arc focused on Ichigo's introduction as a Shinigami and him dealing with his past. The main antagonist was the lackluster Grand Fisher, and after his defeat, many thought Bleach would end, but the plot complicates with the arrival of Byakuya Kuchiki and Renji Abarai, suddenly that elusive "Soul Society" wasn't so elusive anymore. It had become a main part of the plot, and that brings us to the Soul Society Arc.
The Soul Society Arc picked up off the previous arc swimmingly with a seamless transition. The main purpose was to save Rukia, but a hidden agenda the writer had was to bring light to the mysterious Soul Society and Shinigami in the process. After Ichigo and friends gained understanding of the Shinigami and Soul Society (Sekki Sekki Stones, Rukon, 4 Great Noble Houses, Divisions, Captains, Lieutenants, Zanpakuto, Shikai, Bankai, Kido etc.), it was the actual rescue mission's time. Through grueling trial and error, they succeeded in saving Rukia, only to discover that the "antagonists" (Soul Society) were in fact being effectively "played" by Aizen. He then told of his plan and how it stemmed back to the Human World part of the series, so he essentially had been the central antagonist back then, even though we didn't even know of his existence.
The current arc (The Arrancar Arc) continues fluently off of Soul Society and its home is none other than Hueco Mundo. Aizen has established himself as the focus of everyone's anger and has claimed the lives of his own followers (indirectly of course, well, except for Harribel). In this Arc we meet the Vizards, who are an embodiment of the fusion of Soul Society (Shinigami) and Hueco Mundo (Hollow), and establish themselves as another one of Aizen's 'mistakes'. I don't feel like speaking much more of the Arrancar Arc, so go to the article and read up on it.
That leaves just one world left, Hell. When someone asks what's the opposite of Soul Society, a fan would naturally say Hueco Mundo, but that's incorrect. Hueco Mundo is the world where truly lost souls end up, while Soul Society is where pure souls end up, there's nothing connecting them. Hell is where the corrupted, evil souls go to, thus it is the true anti-thesis to Soul Society. Also, Arrancars and Vizards have already been explored, so there would naturally need to be newer heights of power, and if this is the case, then I see the Captains being obsolete if the manga continues beyond Aizen's defeat. If there is another villain in this story, I would bet my bottom dollar that Hell is where that threat will come from, but going by Bleach fashion, this threat won't just arrive as a new villain, it will establish that it was somehow involved with Aizen's scheme etc.
So really now readers, what's next?