really wanting to build a custom marker from the ground up. an suggestions?

[breg]

I really want to build a custom marker from the ground up. I'm not ruling out machining, but it might get expensive. Basically, I want to start with a real plain marker and in the end have something truly unique and amazing. Any suggestions, would really help.
Basically, I'm probably going to have to start wih a Mag or Cocker.