First of all, it sounds like the Adventure Time movie is finally happening! This was announced way back in like 2013 but never really went anywhere. The plot and other elements ended up getting integrated into various season six episodes. Adam Muto, Rebecca Sugar, and Pat McHale are all involved in the project.
Then we also have two series on the way. The first one sounds like it will be a return to the 6-11 demographic, unlike the recent Fionna and Cake series which had a slightly older target audience. It will be titled Side Quests and will be an episodic show focussing on a young Finn and Jake, serving as a sort of prequel to the original show but without many serial plot elements. The Variety article says Nate Cash will be working on it.
Finally, the second series will be a preschool show called Heyo BMO. Adam Muto and Ashlyn Anstee are both involved. I expect we will hear more in the coming weeks. There might be more info at the Annecy festival by tomorrow.
I will of course be bringing more updates as they come, and will dig around for more info later once I have some more time.
Stay tuned!