If the existing fields are not enough for the content element and storing the extra fields in a flexform is not the right solution (the fields need to be indexed for example) then the solution would be to add extra columns to the tt_content table.

This example is not present in this demonstration sitepackage, but since the previous examples already explained the necessary changes in TCA for a new content element it shouldn't be very hard to implement.

The new tt_content fields need to be included in a ext_tables.sql file and the basic field configuration needs to be added to the 'columns' section. By looking at the tt_content TCA file in the core there is enough inspiration; further details can be found in the TCA documentation.