The basic idea in lame ASCII art:
gschem
(schematic) ----> simulator X ---> output X ----\
| |---> oscopy
\----------> simulator Y ---> output Y ----/
....
The simulator output could then be read, displayed and manipulated by oscopy,
which itself is simulator-independent (it's very easy to write a reader for
your favorite simulator output format). Oscopy has an additional advantage in
that it uses numpy so things like doing arithmetic with signals and FFT are
easy to implement.
Within gschem, we could support various simulator-specific flows by having
custom menus that bring up nice GTK dialogs with additional simulator options
etc. The beauty of all this is that most of it doesn't require any changes to
gschem and can be implemented almost entirely via plugins and external scripts.