Which module could help additionally to a SAP MM Consultant for his growth?

The only caveat is that there are many installations where PP is not used and/or is not applicable - e.g. services, project-oriented businesses and distribution businesses.

You should consider SD or PP.

If you have a background and/or interest in manufacturing, PP would be the choice. The only caveat is that there are many installations where PP is not used and/or is not applicable - e.g. services, project-oriented businesses and distribution businesses.

SD will likely be more useful in the market - as all private sector companies, and many government entities do sell things.