Heretical Reasoning
comes to the conclusion that Susan has to come into possesion of the rings.
From this its pretty inevitable that Susan would find herfself in the Wood Between Worlds.. and then from there to another world.
The diversions would come in the form of...
1) The nature of the world she eventually visits
2) The way it alters her perceptions
3) Whether others travel with her.
I must admit that my "What happens to Susan" has been coloured slightly by the song "Oh No, Not Susan" which I recently came across.
/me is currently going through some records unlistened to for a long long long time