Usually a successful distribution meets a previously un-meet need or implements a new feature. Really successful distributions provide new features.
I know you said you wanted to use BSD but If you are just starting out and looking to learn you might want to try Linux from Scratch. LFS is essentially a book with cut n' paste steps that walks you through building a distribution from source code. LFS is mainly an educational tool and many of the concepts of making a build environment with compiler should carry over. I am not sure anything exists like LFS for the BSD's
LFS