Adding to my haphazard style of thought...
Ok...the way I see it, Aizen and his forces are pretty damn strong. Aizen himself is a top-rated shinigami, even amongst the captains. Gin and Tousen aren't so bad from what we've seen and are both captain level. The espadas have immense destructive power and are pretty tough. Not to mention, the other arrancar and perhaps basic hollows/gillians and such under his control. In essence, Aizen's army is immense. If they were to go to war with soul society with the addition of the Vasto Lords, perhaps they'd win.
But Aizen himself...wants to obtain hollow powers and kill the King right? This begs the question...this King? What IS he? The fact that someone as powerful as Aizen seems to think that he needs hollow powers on top of his elite shinigami skills implies that this King or his guards may have immense power...or perhaps, just MAYBE, this King...is the also a mix between hollow and shinigami- a perfect hybrid and to beat him and take his place may require another perfect hybrid.
I mean let's be honest...shinigami that can stand against Aizen are at best-Yourichi, Urahara, Isshin, Yamamato, Ukitake, Shunsui, Unohana and Ryuuken (except he's a quincy). And this is being vastly biased in favour of the 'good guys.' In reality, the list may be much shorter. But Aizen can muster an army which is able to at least slow down these lot to get to the King.
The other thing that quirked my interest was what everybody's favourite inner hollow said...his obsession with becoming 'King' in Ichigo's world. That's the only other time it's been mentioned in the manga, in another context. Perhaps just an inside joke or heavy implication by Kubo that the King may have hollow powers or be a hybrid of some kind like Ichigo.
I know, I know, this borders on wild speculation, hell, it IS wild speculation but I'm only posting here to see if it can be debunked by anyone realistically. For instance, if I missed a vital point which nullifies my entire argument.