I am not sure what use this question is here. Have you thought of asking your users what they want / need. Also try asking around where you are for the percieved pain points with what is currently documented. That should give you a good start.
Read the RoboColum(n) for a tips, tricks and musings on the Technical Communication Suite products.
I'm going to have to largely agree with Colum; your users will be your best source in a way. Unfortunately, the response to "Hey, everyone, we want you to quit bugging us with stupid questions, so we're creating a computer program to handle your more stupid mistakes. What topics do you think we should include?" will not be good. (Yes, I know, I took a LOT of liberties with what you'd say and I don't really think you'd say that. I was speaking more from what the users seem to hear no matter how we say that we're implementing a new system that means they'll be helping themselves.).
Does your company keep a record of the calls and their solutions? If so, that's the number one place to start.
Filter out problems/solutions that the users can't implement. If they can't implement it, there's no benefit to it being there.
Sort the rest by 'most frequent symptom' (as in 'what complaint do we receive the most?')
Then sort the list by 'easiest for a user to fix'.
The more frequent the complaint and easier the complaint is to fix, the more important it is to have on the list of topics.
As a side note: Don't be afraid to include solutions that you think may be just a bit beyond the users' ability to fix. Many users, once they taste the ability to solve their own problems, will be much more likely to go back to your help file and actually feel a sense of accomplishment that they didn't have to call for help when there was a problem.
OH, and when you implement the help, make a policy for your help people based on the topics included in the help. If the symptom is in the help file, then the policy should be to direct any call-in user to the help and walk them through using the help ("Click on this topic.. now see those steps on the right hand side? Those are how to fix it. Would you like to go through them together, or do you think you can handle it?"). That will get them more comfortable with the idea of online help (biggest block to using online help: I'm not sure I can figure out the problem and fix it and I want someone there to help). Have your people nicely and politely hammer that home. At first you'll spend a LOT of time walking them through, but it should drop as they grow more accustomed to navigating the help and working through the steps.
Use the first few months as a shakedown, too. That's the best time to ask of certain steps aren't clear or if the layout isn't logical.