It's not Aizen's Plan (it's Urahara's)

Inspired by my conversation with . Seriously, the more you think about it, the dodgier he is.

We know that he was researching the same things as Aizen, namely how to combine shinigami and hollow reiatsu to allow beings to become more powerful. I realise this doesn't incriminate him; I just find it a little suspect. I mean, they were colleagues at the time, so did they never consider collaborating on the project.
He created the Hogyoku.
He knew about Isshin. He knew about Isshin before the first chapter of Bleach, yet he still let everything unfold as it did. Think about that.
Based on the above, it seems highly unlikely that when he sensed a shinigami (Rukia) tracking a hollow and going to the house of a shinigami-human crossbreed (Ichigo) that he was not alert and aware enough to step in before the sh*t went down.....
..... In fact, his timing e.g. just too late to prevent Rukia's transfer of powers to Ichigo but just in time to save her was a little suspicious.
He stuck the Hogyoku in her body. Without telling her or anyone. And put her in a gigai that would have eventually deprived her of her powers. All this, knowing that Aizen would probably come looking for the Hogyoku.
He seemed to keep a pretty darn close eye on Ichigo even when he was training with Rukia.
When Rukia was arrested - slightly suspicious timing once again. Far too late to prevent her arrest, but just in time to save Kurosaki. Funny that.
He did some very odd stuff to Ichigo in his training, including causing (whether purposefully or otherwise) his partial hollowfication.
He claims he can't go to SS with Ichigo because the soul-reapers have prevented him re-entering, but later has very little trouble when, at the end of the Aizen arc, he travels there and eventually defeats Aizen.
He knows about the visoreds. Not even a little. They used to all be soul-reapers together. So why doesn't he tell Ichigo of their existence? Why doesn't he explain that's essentially what Ichigo is, and why doesn't he help out because he was a scientist researching exactly that stuff when he was in SS.
You know how he knows Kuukaku and Ganju. Well, isn't it a little odd that they don't comment about how much he looks like their own brother, even though everyone else notices. Well, now, we suspect that Ichigo and Kaien may well be cousins, what are the chances that not only Urahara knew, but that he also told Kuukaku and Ganju to expect him in advance.....
Which also makes it a little freaky - if he knew that Kaien and Ichigo were related - that he picked Rukia (Kaien's killer) as the pawn he sent to SS with the Hogyoku inside her.
After ALL the levelling up, Ichigo still didn't defeat Aizen. That was Urahara. Ichigo just bought him time. So..... well, I guess that's not suspicious, but scary-powerful, and also, he still has the situation under his control, it seems.
He and Isshin are pretty pally throughout the Fullbring arc. Again, he knew what was happening, but decided not to enlighten anybody whom it actually helped, which, in turn, meant he stood back and watched Ichigo attain new levels of power. [I think Isshin may be in on all this too].
He's pivotal in convincing SS to give Ichigo his shinigami powers back.
He used to be the guardian of the Maggot's Nest, a prison establishment that, by its own admission, imprisoned people considered dangerous to SS because of their powers or their ideas. Their ideas? Now, I'm not blaming Urahara for SS's conservative ideologies, but for someone with that background, he's really rather ott on the "think for yourself Ichigo!" I can only assume that he has turned completely against those old ideologies, which is interesting in itself and makes you wonder why he is pushing Ichigo into a position that may put him at odds with SS.
The only stuff we knew about quincies was what Ishida himself knows and the history that Urahara told Rukia. Given the current situation, should we trust his version of events? He's proven pretty good at hiding information if it suits him. He said quincies were human. I'm not saying they're not, but they're doing a pretty good job of lurking about in the afterlife.....
In the most recent chapter, we find out he's analysed water from the Soul King's realm. So, has he also been to the Soul King's cities? Why is he so 'in' with Zero Division that they are sharing their magical healy water with him.
I bet he knew Ichigo's mum (he knows Isshin well enough) and could have told Ichigo whatever is about to be revealed to him by dodgy quincies. [Isshin is definitely in on this].

All of this is circumstantial and does not prove he's evil. What I suggest it does show is that he appears to be motivated, not by a desire to help out (he has withheld information and not helped in situations where he COULD have), but by a desire to see exactly how far he can push Ichigo. Assuming Ichigo is Shiba clan, a side effect would be that he is restoring their name within SS..... which might also be interesting.