You can be a SAP developer and build addons/customizations.
SAP has an entire developer community, etc.
Implementing SAP, full-cycle, however, is another issue.
The fatal flaw that too many SAP implementers have is they ruin the competitive advantages of a business and replace them with SAP processes, instead of modifying SAP systems to magnify the "how we do it here".
I ended up overseeing SAP implementation without signing up for it. Ended up doing it full-cycle and learning as I went. No holidays, no christmas, lots of time away from family and friends, and well over budget and deadlines..
The best thing is when it actually works. The hard thing is the assumptions made about how a company does its business and whether or not processes can be arbitrarily changed to the convenient, simplified understanding of the developer. I am not sure yet which is greater, pleasure or pain..
Without a complete analysis, and technical spec with an understanding that everyone comprehends the blueprint the same way, it's a tough thing to take on. I see now why so many businesses fail because of an SAP implementation.
If you want to stand out, I would say get very, stupidly good at learning how a client's business works. From a high level, right down to the nuts and bolts and how they interact.
The biggest flaws of SAP implementations is a failure to understand the details, the implications of how the details interact, and which details are important. Like 70% of software projects that fail, this is usually where the greatest hurt from SAP comes from. Be a master of this and you will rule.
For me, I get requests to oversee SAP projects. Some things I wouldn't do for all the money in the world. I have a great deal of admiration and respect for the very best SAP implementers. It really is a path of continuous improvement and learning. It can be a very draining (or rewarding) experience.. usually both. Where consultants do not marry their client's success, the relationship isn't as strong. That being said, I wouldn't turn down everything, it would have to be a company I was passionate about enough to take on something like that again.
Hope that helps. I would build add-ons if I were you. They are profitable, and your scope is nice and limited from the full-cycle fun and it will establish you as having some value. Whether or not you sell it is a secondary thing. It can go on your CV and people are more inclined to hire you because you have invested yourself in it.
Best of luck!