Can I develop external Apps to a company with ABAP out of SAP software?

I am a recent graduate in Systems Engineering (with a symbolic 2 years work experience in my area).

I'm loving the programming, development (web, mobile and desktop) applications.

I'm thinking of doing the certification SAP: ABAP. My question is: after obtaining the certification, Can I develop something more besides SAP basis Modules with the knowledge provided by the course and certification? I mean, Can I develop an App XY for an company C even if this one dont have any module like Basis, Bi, etc?