Nintendo of Europe recently posted an ambiguous Tweet which may have hinted at more Mario Kart 8 content coming soon.
In addition, there appears to be a picture of racers crashing on the Mario Kart 64 track Kalimari Desert. With E3 on the horizon, it is no surprise that Nintendo plans on making big announcements soon. Maybe it will be the hidden track that was recently discovered.
This Tweet may hint that Nintendo is planning on adding more retro stages for players to race on. Kalimari Desert was also remade for Mario Kart 7, so it may point to Nintendo bringing back popular maps.
The latest DLC for Mario Kart 8 was released in April 2015 where Nintendo added a new mode, 200cc, to increase the difficulty level of gameplay. Prior to that, Nintendo added new characters, tracks and karts, so only time will tell if Nintendo will add even more new content.
Hopefully, they will add a "death stare" for each character.