It would be nice to have an extra field in the personal information area of a client for specifying a title.
This should get rendered in the invoice pdf in the billing to area:

Mr.
Christian Bauer
Example Street 1
Example City
etc.

The extra field could be a drop down for common titles like "Mr" or "Mrs" but also should provide a free form text field in case one would need to specify a complex title (including academic title etc).