what I do now alot when I am teaching 'set' techniques is to define what uke should be doing (pushing, pulling, stationary, running, lifting their elbow - e.g. when trying to resist etc). Therefore nage gets used to the feel of what is weak about ukes attack, and can exploit this weakness with an appropriate attack. Therefore, when it comes to randori it is not a question of nage thinking 'now I will do ikkyo', but of their body realising where uke is weak (due to ukes movement or response) and exploiting that to carry out a throw/technique. To me this is the big difference between Aikido and ju-jitsu i.e. aikido is purely in response to uke, and is not forcing a particular technique on.