It would great to have Global Variables section in addition to per snippet variables. In my case I'm reusing variables across most snippets. Instead of adding same variables to each snippet It would be very helpful to have ability to set all variables in one place (Options) and reuse them across multiple snippets. Needless to say if snippet has its own variable with same name it should ignore global one.
That would be outside the scope of how Snippets is set up. That would require a separate tag to set those global variables before using any individual snippets.
You could use Sourcerer to set global variables in PHP.
Then in the snippets, use PHP (also through Sourcerer) to output those globals (with optionally some fallback to a default value).