Interesting thread. Isn't it always the basics that are the most elusive?
As a foundation for aikido, I would agree that aiki is pretty much it. The way to join ki with the partner. Or another reading: the way through ki to reach unity.
For the aikido practicioner, though, I believe that the center, tanden, must be the very root. My first Japanese teacher, Ichimura sensei, stressed it all the time: Be in your center, do it from your center, return to your center...
I say the same to my students, so much that I fear they hear it in their sleep.
In any budo, one needs foremost to awaken one's center, and then to let it be the origin of all. Techniques are born from the center. The beginners need to copy such expressions of the center, to awaken it in themselves.