The brain of a teacher is a goldmine of knowledge, creativity and micro-management. Knowledge of one subject is one thing and delivering that to the audience (like students) is a challenge which not everyone can do. Therefore I think teachers should share their knowledge with online audience as well. There are several topics that teachers can write on:

1. Their own subject:
A Physics teacher can write on topics like gravity, inertia and more in a way which is simple to understand and not exactly the technical way it is written in the text books. Teachers experience challenges in class and they know exactly which points need examples or elaborate explanation in general.

2. Discipline Articles:
Regardless of the subject you teach, Discipline is a challenge which every teacher faces. The problem increases as you teach students of higher grades or a topic which is hard to understand or which students may not like. Maths and Science teachers face these situations in every day life.

3. Micro-Management Articles:
Teachers have tackle each and every student personally. There are parents and other teachers who may benefit from the knowledge and tactics used by a teacher who can then use those ideas themself. Benefits of writing articles about discipline and micro-management can be very useful.

Well these are some ideas on which teachers can write articles online.