Yes, it's been done before, but in order to pull it off properly, the first main character must disappear out of the storyline either for good or for such a significant portion of the time that the old main character is no longer relevant to the new story arc. Furthermore, there must be something with the new main character that makes it more interesting over the old one. Failure to do so will only produce a "but it's not like character X!". Which to be honest, is the most common result when someone tries to change main character completely, especially without a clever plot device (the perceived main character was in fact somebody else all along or something like that).
The problem in particular is that when a main character remains a main character for such a long time as in Bleach, people really start to root for the main character regardless of it's liked or not. The main character shapes the story. Changing it too suddenly, which it would be in this case, will just act detrimental to the story itself. Simply bad writing. Personally, I really root for and like Ichigo and wouldn't want him to go away. For me, Ichigo IS Bleach. The most annoying and recent example I have when an attempt at switching main character was done (and particuarly so without it being necessary) was the second season of Darker Than Black. Instead of rooting for the new main character, fans of the show just kept longing back to its days when Hei was the main character, and he furthermore kicked so much more ass. He was simply better written and far more interesting, all in all. Furthermore, Hei made an odd cameo in the second season, just strengthening the viewers' wish for a change in narration.
So tl;dr
If Bleach would switch main character from Ichigo to something else, I'll quit reading without any form of second thought. Bleach without Ichigo is like X-Files without Mulder. It works, but never as good as it was before the change.
By the way, that dude from Gantz, wasn't he the one who was a fucking crybaby although he was supposed to be the one acting all brave?
And great idea of a plot twist there, daz bonez. I find it unlikely though, we need an Aizen break, honestly. I think most of the reason that dude looks like Aizen is because the way he smirks in that particular scene. In the other scene when he is chasing the thief he certainly looks more like Isshin. Plus, he got a somewhat similar hairstyle to Aizen.
EDIT
Just to actually add a bit on topic (I'm in a Bleach rant atm if somebody noticed, probably because I've been away for weeks

),
Mulzen as the other Chaos with an avatar named him, might rather be an independent portion of
Aizen. Remember the part where
Aizen said that he lost his zanpakuto? That is
Mulzen, manifested into a human shape in order to main its existence in the human world in which it was spawned. This doesn't imply that
Aizen got no soul, since
Aizen IS Aizen's soul, but his soul split in different parts. Since the zanpakuto is the representation of a shinigami's reason to protect and
Aizen lost this reason somewhere during his fight with Ichigo, it is therefore merely a part of
Aizen can thus be separated. It seems that
Aizen may even have consciously wanted to get rid of his zanpakuto for unknown reasons, but it seems odd that one should be able to vanquish specific parts of a soul. Therefore the possible solution is a split, given that the hogukyoku only works within the limits of probability.
Now, what if
Aizen's reason to protect is somehow linked to Isshin's past? Now you have a perfect set up to both introduce and flesh out
Aizen as a character, as well as Isshin without actually having to do much with the actual
Aizen at all, as
Aizen's zanpakuto is not
Aizen himself. What
Aizen's zanpakuto does is thus not strictly related to Aizen in any way, but can serve to introduce
Aizen's (necessary) comeback.
Brooding further on my old psychoanalysis, I am also thinking that since
Aizen is already a shinigami, he must be subcategorized further beyond being a super-ego. Specifically, I think what I am looking for is "reason".
Aizen's zanpakuto acted as his reason, his ability to logically deduce and act, well, what we previously thought was typically
Aizen-ish. Why reason? I base this on the idea that
Aizen actually went rather insane at the end of the last arc. It was a slow development, but it came to that at some point. After being judged,
Aizen still seemed to be pretty much the same, not necessarily acting insane-insane, but at least strongly expressing a feeling of being misunderstood and misjudged, along with a hefty amount of anti-authoritarianism (which is common for a majority of all important Bleach characters). His former self-control seems to be completely gone, as shown when he mocks the judge of his sentence.
Anyway, it makes sense at some level. I might be entirely wrong, but at least it's a theory and a rather thoroughly one

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