Just a few ideas, probably change my mind on this in the future:
ikkyo has much of the essence of aikido because
i. ukes hands are directly related to their intension (i.e. if they block you can do ikkyo on the blocking hand, if they attack, you can do ikkyo on their attacking hand)
ii. the extension of ukes arm is directly related to the control of their centre, and the over-extension of uke (as Ueshiba said, anyone is powerless outside their own circle of power - or something like that).
I do not think it is just coincidence that ikkyo is also an important technique in kung-fu/tai-chi push hands excercises. The interplay between extension and over-extension will effect who does ikkyo on whom.
However, although I think a complete grasp of ikkyo may give you a complete understanding of ikkyo, I would consider that all the techniques in aikido make one whole - there are no 'new' techniques to be discovered nor techniques which can be removed. I've only realised recently that there are not really any seperate techniques (and this isn't philosophical bullshit) and that these techniques just appear out of no-where if you are keeping the balance between yin & yang.