To discuss something means the same thing as to talk about something, so the word 'about' is not correct to use with 'discuss'. So, to go through your examples (removing 'about'):
- During the meeting we are discussing XYZ.
Incorrect. The meeting is in the future, and "we are discussing" is present tense. You can't use the two together.
- During the meeting we are going to discuss XYZ.
Correct. "We are going to" talks about the future, and so is appropriate here.
- During the meeting we'll discuss XYZ.
Correct. We'll is a contraction of we will, which is also future tense and appropriate here.
- During the meeting we discuss XYZ.
Incorrect. We discuss [x] refers to a habit in this case. If you have a recurring meeting in which you always discuss the same thing, you could instead say:
- During the [weekly, daily, etc.] meetings we discuss XYZ.