I would be interested in contemporary literature and music of both Spain and Latin America, politics, turistical attractions worth to be visited.
on the grammatical side:
-verbs with prepositions (like phrasal verbs in Englisch) por/para
-use of subjunctive vs. indicative mood
-direct/indirect speech
-more difficult quizzes in the premium section
-maybe preparing for language tests, like the DELE of Insituto Cervantes
-regional series for the intermediate content (I know it's a lot of work and you won't have to realize it all in the near future
- don't forget the the Europeans, maybe a Core curriculum lesson by a Spanish native speaker from the Iberian Peninsula in the future would be great