If you follow Nintendo of America on Twitter, they've been highlighting a single track from Mario Kart 8 every two days since August 8th:
8/8: DK Jungle
8/10: Dolphin Shoals
8/12: Moo-Moo Meadows
8/14: Yoshi Valley
8/16: Cheep Cheep Beach
8/18: Bone Dry Dunes
8/20: Donut Plains
At this rate, they'll have highlighted all 32 existing tracks by Oct. 9th. If there is DLC coming this year, I expect they'll announce it then (and maybe even release it since 10/9 happens to be a Thursday).
I think October makes sense since it would come after Hyrule Warriors, but before Bayonetta 2, Captain Toad, and Smash Bros., which I think are supposed to release between late October and late November.
Hope I'm right!