I tried myself to formalise aikido, and thought of all the techniques I could (about 60 or 70, multiplied by irimi, tenakn, soto, uchi variations). However I then realised there are lots more variations on each technique, and it is not appropriate to consider irmi and tenkan variations for all techniques. It was then that I realised that the techniques do form one whole. In my mind their are 3 core techniques:
-ikkyo (from which derive all the pins)
-irimi-nage (from which derive all the throws to the head)
-kokyu-nage (from which derive all the throws on the arm/body)
(NB this is not quite the way Tomiki formalised it).
However I believe this is the core. (although shiho-nage is an odd-one, and may form a seperate group).
If you watch Ueshiba on video he almost always does ikkyo or irimi-nage. Learn these two and that is a good basis . (I don't think its any suprise that the common aiki-jitsu excercises are based on ikkyo, irimi-nage and shiho-nage).
Next thing to do is to learn to blend with your partner (i.e. when they pull, move in, when they push (or strike) tenkan).
(N.B. the soto i.e. under the arm movements occur when a gap appears under the arm, often a grab with strength or a sweewping cut).
Finally, I am convinced all techniques are decided by uke - everything else is just force.