It was tweeted on the official 15th anniversary twitter account that another project is happening.
They also tweeted an image.
When the tweet has been translated it says [announcement] Adventure is starting to evolve further ━ new project!
So I guess we will have to wait until more information has been released.
What do you think it could mean? a sequel to Adventure or something else?
Edit: Toei-Anime US has tweeted an English version of the image here.