I honestly believe that Soul King/Gotei 13 will be the final villain(s), even more: I see Aizen and Ichigo (and possibly Vizards) teaming up against Soul Society, Royal Guard and Urahara (who strongly believes that King can not be replasced) in order to install a new regime over the world.
"Why?" many would ask, "The king has done nothing at all!"
That, ladies and gentlemen, is the who~le point. Could you imagine what would happen to a successful, democratic/totalitarian country where a president suddenly packs up, boards a plane to Hawaii for indefinite time and leaves the country in the hands of some greedy bastards who care for nothing but their own well-being? The result would be pretty much the current state of the SS, no? Militaristic center and the rest of the state reduced to slums where authorities are not even trying to fight starvation and crime. People starve, get beaten, raped, murdered just because nobody gives a crap for them. It is only natural that certain people who have some kind of potential, like Aizen in this case, would strive to make a difference, introduce a little change. The man starts acting: he has perfect ability for ensuring the success of his plans (Kyoka Suigetsu) and he gathers a couple of followers, who, as he is sure, will not betray him immediately. After that he devices Hogyoku and eliminates the only threat in Soul Society (Hirako Shinji) as early as possible. 100 years later Aizen openly challenges Soul Society and starts a war, which would have been successful if not for 2 factors:
1. Arrogance - not much to explain here, let's just say he would benefit much more if he actually treated his allies like they were more than mere shit and tried recruiting people like Ishida and Ichigo. (At least the former would have joined him eagerly)
2. Aizen was
soft! Yep, he most certainly was. How many of the Soul Society captains and vizards died in the war? Heck he could have slaughtered them with no problem, the guy slices through Bankais like hot knife through butter. The dude had a clear plan: Become the king and reform the system
without plunging SS into chaos. I mean, really, imagine what would have happened if Aizen had killed them: Yes, he would become the king, but, seeing that most of his intelligent arrancar were dead, he would lose control of trillions of people, and even more or less orderly rukon regions would have succumbed to total chaos. The soul burial procedures around the world would have been seriously messed up without proper coordination and god-only know how many people would have ended up hollow. My point is, that while Aizen mindfucked people he still left them alive, he was soft on them, which, ultimately led to his downfall. I mean, what was keeping him from travelling to real world and stabbing Urahara in his sleep? The only one Aizen did want to kill was Yamamoto, for obvious reasons.
My overall point is, Kubo strongly implies that Aizen is not your everyday mad terrorist.
Now, Aizen is sealed in an underground prison and probably f*cks his own mind trying to figure out where he had gone wrong

Let's leave him there for a while
Now to Ichigo.
The current arc obviously serves as a device to give Ichigo yet another ridiculous power-up: Fullbring. Just imagine what kind of monster Berry-tan will turn out: Vizard with a fullbring

He most certainly will, in terms of raw power, become on par with captains like Shunsui and Unohana. I mean he will have Bankai, Mask, Ressureccion (no way Kubo will drag manga for 10 years without Zangetsu and Hollow Ichigo) and some pretty broken Fullbring - That is no joke. (Here goes wild speculation

After the end of the curent Fullbring arc Ichigo, in some way, will come into conflict with Urahara and Soul Society due to some sassy reasons. I honestly see Ichigo going "WTF, where are my friends going to end up when they die O_O" at some point during the confilct. He will, of course, act like a total idiot he is, and Central 46 and Gotei 13 will outlaw him. Seeing that even with Vizard's and his "nakama's" help he won't be able to compete with a combined power of entire Soul Society (Yama ftw) and Urahara, Yoruichi and Tessai (perhaps even Isshin) Ichigo will arrive at a single logical solution: "must get a help from an immortal guy who has already put the bastards in their place".
Back to Aizen-samaaa! yaay!

Ichigo will rescue Aizen from prison and together they wage war on Soul Society and, conseequently, Soul King who will turn out to be either a total douche or a complete impotent.
Whew, that was quite a rant

and a bit of wild speculation

Still, I believe that this will be general course of manga in the future!
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The dude said that Aizen was his favourite char, so I guess that there is some room for even slightly accurate speculation
