This definitely crossed my mind, and I've thought about it quite a bit. I guess it's really left to speculation, and only Joss can truly know.
It makes sense that she could have been a Slayer, as their backstories are so similar. However, it makes as much sense to say that Joss simply wanted to show that sort of link between the two, a comparison. Either way, the episode is much more meaningful than it is at first glance.
Both were chosen, both have a mission, both are there to protect and save their people. Inca and Buffy wanted normal lives, lives they were never allowed to have because they were chosen. Both fall in love with different types of beings (Princess Inca was a mummy when she fell in love with Xander, a regular human. Buffy the Slayer falls in love with Angel the Vampire). Both loves are doomed from the start, and their missions are meant to come before themselves - they are expected to sacrifice their personal happiness for the greater good. Inca, however, abandons both love and duty for her own survival, whereas Buffy sacrifices her love and herself (and her chances at a normal life), for the world. Just as Faith later appears as a shadow of what Buffy could become. I think Inca shows that as well. Throughout most of the episode you feel for Inca, you wish things could be different for her. However, when she finally shows that her own survival means more to her than anything else, we truly see the selfishness and evil in her. After watching that, I think viewers gain more of an appreciation for Buffy - she fights the good fight, and makes the choice to continue.
