I think the reason for Aizen's interest in Ichigo is simple:
The whole "Destroy Karakura Town" plot is just a ruse to distract everyone from Aizen's real goal: Ichigo.
Aizen doesn't need thousands of souls to create the Ouken, just one special soul, which would be Ichigo since he's something like highly condensed darkmatter reiatsu. Did you notice how Aizen was feeling Ichigo's dark reiatsu as it slipped through his fingers? That's just him checking to see if Ichigo is ripe for the harvest, aka Ouken time.
How did Aizen learn of Ichigo's existence? Simple:
When Aizen was sifting through the confidential archives in SS regarding the Ouken, he probably spotted something regarding the use of special reiatsu to recreate the Ouken which manifests once every century or so not in an area like Karakura as the shinigami had thought, but in one person, Ichigo.
Notice how throughout the beginning of the series, everyone around Ichigo seemed to gain heightened spiritual awareness or abilities like Inoue and Chad? I recall Ishida mentioning how Ichigo simply "oozes" reiatsu affecting all around him, even amplifying Quincy powers merely by contact. I don't think this "amplifying" characteristic applies to anyone except Ichigo, which is his defining characteristic. Central 46 probably knew that Ichigo's characteristic would show up every few generations and was constantly on the lookout for it due to its Ouken potential, which may be related to Isshin and why he left SS. With Aizen scheming to create the Ouken WAAAAAY before he even became a Captain, he probably caught wind of Central 46's concerns regarding the characteristic "reiatsu anomaly" ie Ichigo, and focused on areas with high spiritual concentration, and then pinpointed Ichigo after his first encounter with Rukia when she used a binding spell on him, which he broke through despite being just a "human".
Basically, all of it, as Aizen said, was leading up to this pivotal point in the series. So yeah, it's "Strawberry" harvesting time.