Previously i have mentioned some obvious areas which can be looked into where you are likely to pick some projects. Here are some more ideas which will guide you towards your continual improvement journey and six sigma project selection.
To build the culture of excellence through continual improvement, you need to start at some point and show some results. One good project completed will get push from the management to believe that continual improvement must be the strategy to grow. It can be used to scale up and bring in more value to the stake holders. If the strategy includes involvement of people, this will spread like a wild-fire across the organization. Here are some ideas based on issues currently faced by the people in the organization. As a mentor, i would suggest to start one of these ideas for six sigma project selection to prove the results in short-term and get the buy in from the stake holders.

Efficiency/Capacity Issues
Processes producing less than expected
Processes requiring overtime not requested by a customer
Processes requiring cycle times in excess of expectations
Processes with computer/machine downtime
Processes requiring expedited shipping and associated costs due to delayed output production
Any process that has added labor to make the required cycle
Too much of waiting between hand offs
Call drops or disconnects during customer calls.
Low customer Satisfaction

Understanding Process Variation
How much variation is there in your incoming materials and/or process parameters and how does this affect your output?
Where do you need the input controlled to always have a good output?
Can you scientifically adjust your process to compensate for changing material, weather, etc.?
Can your Black Belt help your supplier do a project to control the incoming product where you need it?
Does understanding your inputs allow you to produce a good part using less material?
Time taken for each call by various employees
Time taken for invoicability of the resources.

This list can go on. Essentially the power of Six Sigma is really great and can be applied across all issues to be solved. However, there is no point in making a project when you have obvious solutions visible. If you have obvious visible solutions they shall be implemented before taking up a six sigma project.