There should be a Basque option. Medieval Spain was very Basque influenced which is why Spanish doesn't sound French-ish like Catalan and Portuguese.
Favorite would be Roman Spain, then Democratic, then Basque, then Castilian, then Moorish, then Imperial, then Trastámara, then Celtic, then Visigothic.
Protestant Spain would be interesting though. As would a Viking Spain. Or maybe a Bantu Spain.