So ... aiki.
What is it?
Have you felt it?
Can you do something you think might be aiki?
Do you have the means to develop it?
Are you searching, or remain just content to be 'told'?
Have you sussed anything interesting?
Have you discerned any interesting principles that you can apply across a range of waza?
Can you move people that resist?
Can you take people's balance with subtle craft?
Or ... are you just training and hoping one day ... It'll just happen?
Oh... great questions, Rupert!
You're right, "aiki" is very much a learnable skill.
It results from a very specific kind of body training. It has nothing to do with martial technique, but it is the powerful engine that will drive technique of any kind.
There are no "aiki-like" katas. Training practices either are designed to promote a process of specific movement and structural manipulation to produce IP and aiki, or they are not. The process involves learning how to use mental intent to manipulate unconventional muscle groups and connective tissues to create a dynamic tension of opposing forces within the body -- the In/Yo (Yin/Yang) that are in a constant state of change from neutral to varying stages of imbalance and re-balancing that constitute the "harmonizing of ki" ... Ai-Ki. That's what makes the dynamic tension, the potential energy which is the source of power that is directed to the desired use by the intent and will of the mind.
Anyone who is training in this very real method, does not have to guess that they have aiki, what it feels like to "do" it, or how it feels when in contact with someone who is expressing aiki. They are training in a curriculum that was designed for aiki development. The training is quite specific. Aiki will never "just happen."
It would be great to see it reintroduced into modern, mainstream aikido. Some people already are doing that, having sought out and found sources for acquiring the skills. I don't believe that it ever will be taken back into all aikido, lock-stock-and barrel, but I do think that there will be a small-scale revival of aikido that at least in part preserves and reflects the skills that Ueshiba continued to cultivate and refine till the day he died. As long as even a small pocket of those practitioners exist, there will be an aikido that is the Way of Aiki.