I would disagree with you there - I would actually characterize the Eragon/Arya subplot as 'filler' (and poorly planned as well, since their potential relationship is based on, essentially, nothing) between the major plot of Eragon Being Awesome (TM).

Although I have to agree with you that it's unlikely that Eragon will actually fall for Nasuada; I simply would be very pleased (and impressed with CPao) if he did.