Leaning the mixture (both how to, and when to re-enrich it.)
Side-slipping.
Both the above now covered (after getting the licence, I booked some time with an instructor specifically to cover these.)
To whom to talk on RT: generally covered in CAP413, I would have thought, though often only by implication. I think a useful section of training would be ground briefing to cover some likely cross-country scenarios from this point of view.
Some early instrument panel familiarisation, and revision/more items, as training goes on would be useful. For example, I was initally unsure about how to use the intercom panel to select what I wanted - some reassurance would be useful, and I'm sure it would make a difference for early students.
In general, though, I've been very happy with the training I've received so far: and have had several different instructors: this has enhanced the experience for me, gaining some experience through their slightly different way of seeing things/explaining things.