Re: What do you REALLY want to teach?
My teacher thaught me it is important that each student takes an interest in the lesson. Off course each at their own level. Not everybody might be able to incorporate all the principles in each technique. For instance you might notice that ma-ai is a problem for most in a particular lesson. You'll have to think about what exercises might improve that. This process makes you a better teacher and improves your understanding of Aikido. And, your students improve their sens of ma-ai (in this example).
I always start with kino nagare exercises and check how the students perform. Next I decide what aspect to work on and how.
As the lesson progresses I adapt when/where necessary.
The scariest part has been *not* to prepare the lesson and react to the group as the lesson progresses.