Last month, it was revealed that the 2020 edition of the Dakar will take place in Saudi Arabia, marking a shift away from the South American continent for the first time since 2007.
It will also signify the second year in succession that the rally-raid has taken place within the borders of one country, after this year's event was held entirely within Peru.
Although Saudi Arabia has a five-year agreement to host the Dakar, Castera says the rally's organisers, the ASO, are already looking into which other countries could join the route in 2021.
Jordan - which borders Saudi Arabia to the northwest - is considered the main contender, although Egypt is another possibility.
"[The agreement with Saudi Arabia] is five years, but in the second [year] it will open to other countries," Castera told selected media, including Motorsport.com. "Only the first year will be 100 percent in Saudi Arabia and then we will evaluate other countries too.
"For example, there is Jordan. I've been talking with their representatives these days and they've asked me to go there. In other words, there are countries that want to enter and we have the opportunity to do so. Egypt is very close [to Saudi Arabia].
"It is true that it is not so simple, but it is clear that there are other countries that can enter".
Hosting stages in Egypt would involve the Dakar crossing the Gulf of Aqaba to reach the Sinai region, or crossing the Red Sea to the south.
Egypt previously hosted the rally in 2000, when the rally began in the Senegalese capital that gave it its name and ended in Cairo, and in 2003, when Sharm el Sheikh temporarily replaced Dakar as the event's finishing point.