The most natural progression is probably CRM. Customer Relationship Management processes are basically extensions of Sales and Distribution processes. Having said that, there is significant overlap with most of the modules you listed. What it boils down to is your own interests. I suggest you do quite a bit of research on all the modules you listed and find the one that sparks the most passion in you. I have to say, though, SD, in and of itself, has plenty of depth. You don't actually need to diversify to enhance your career. There are several ways to specialize within SD that will keep you eminently employable. The key is to be passionate about what you do and strive to be the best.
Hope this helps!
Best regards,
--Tom
