I would love to see a series based on the history and evolution of the Spanish language. SpanishPod has many great lessons focused on practical uses and also cultural/regional experiences. I know the majority of Spanish learners would gravitate to the 'must-know' topics at first, the practical topics. But I find that as we learners progress into a more advanced stage of Spanish, there comes a fascination with the language itself. I think it would be really enlightening to know more about how Spanish came to be.
I saw this touched on in a few of the lessons -- there was one lesson that covered why Carribean Spanish has a certain accent inherited from Andalusian Spanish, another one traced the historical reasons for the usage of Vos in some areas and not others, and another that mentioned how the double "ll" once were pronounced in the past in Spain. I found these little tidbits really fascinating! If anything, they gave sense and context to things we may have already noticed.
Given the linguistic forte of your awesome hosts, I'm sure this would be up their alley