I think this proposal is unlikely to gain traction. But if it were, in a Lisp context colon (:) would make more sense IMHO, as it is used in Common Lisp to separate the namespace ("package", in CL-speak) from the symbol name. That would make easier to adapt to CL-style namespaces, if they were implemented in Emacs some day (which I think won't ever happen, if previous discussions on the subject are to be believed).
I just want to voice my 100% agreement that the minus character should be changed. Its double use is, as mentioned in the original post, incredibly annoying to read. In my package, gnu-apl-mode, all symbols are prefixed with gbu-apl- and it's full of these issues.
Whether the character is / or : or anything else, I could not care less. All I want is a symbol that isn't much used for anything else.
The benefit of / is that a lot of third party packages already use it.