Consider the following before writing:
* the initial conversation between Ganelon and the Saracen envoy.
* the way in which Ganelon states Charles's message (in the harshest way possible).
* How Blancadrin assists Ganelon in directing anger away from Ganelon and Charles and instead directs it towards Roland.
Naturally you will want to discuss Ganelon's motives, but they should be mentioned in the opening paragraph.
Here are some ideas, modifications and options:
Begin with the talk between Ganelon and the Saracen envoy Blancadrin during the journey from Charles' headquarters to Marsilion. Then Ganelon states Charles message in the harshest, most offensive terms possible, and, acting as his surrogate, draws Marsilion's hot anger on himself. But with Blancadrin's help, this anger is diverted from Ganelon and Charles and directed at Roland. They now plan the destruction of Roland by means of an attack of the rear guard.
Naturally you will want to emphasize the bad relationship that existed between step father and step son, because this must have played some role in Ganelon's motives.
Other possible thesis assignments you may wish to write about are the following:
The attitudes towards Islam in The Song of Roland
Oliver's attitude towards Roland
In what ways is the Song of Roland a demonstration of the reverence towards kings in the Middle Ages?
The supernatural and the super human elements in the story(omens, dreams, the angel, and superhuman efforts).
Anyone want to cover Ganelon's trial?
If you choose one of the above (new) thesis choices...create your own title.