http://mynintendonews.com/2015/03/02/next-mario-kart-8-dlc-is-in-the-final-stage-of-development/
Translation (Google Translate):
[For Customers in Japan]
also from mariokart.tv, I can now purchase additional content!
The first installment, "The Legend of Zelda: × Mario Kart 8", is currently being delivered. Those who have not yet been played, please try by all means to this opportunity!
2nd "Animal Crossing × Mario Kart 8" is the delivery scheduled for the end of May. Production is so has entered the final stage. Please be patient.
Possibly a Nintendo Direct soon showing off one course?
[For Customers in Japan]
also from mariokart.tv, I can now purchase additional content!
The first installment, "The Legend of Zelda: × Mario Kart 8", is currently being delivered. Those who have not yet been played, please try by all means to this opportunity!
2nd "Animal Crossing × Mario Kart 8" is the delivery scheduled for the end of May. Production is so has entered the final stage. Please be patient.
Possibly a Nintendo Direct soon showing off one course?