Ojamajo Doremi/WMG
Momoko will name her future child after Majomonroe.
Although Doremi and Hazuki didn't go to the same school at the end of the series, they will go to the same high school in the future.
Bonus points if it is Sakuragaoka High.
When a witch dies, their fairy goes with her.
It would explain why the fairies of deceased witches never appear. Could count as Fridge Horror.
Hana-chan will adopt the name "Majohana" after her Awesome Moment of Crowning.
Since "Witch-Flower" sound a whole lot more dignified than "Flower-y".
The door to the Witch World eventually reopens.
All of the girls made promises to see each other again, and Doremi does have a granddaughter who eventually becomes a witch apprentice.
(The following should seem better fit under Grand Unifying Guesses, since they crossover with unrelated series.)
Hazuki and Momoko are the ancestors of, respectively, Tsugumi Rosenmeyer and Silvia de Alisia.
They're probably cousins. It's easy to tell: not only both series have character designs by the same person (Yoshihiko Umakoshi, who also did Casshern Sins), Doremi and Tsubomi both wear pink and their hair is the same shade of red. Heck, if we think about it, both series might even take place in the same universe.
Non-chan is Charlotte.
Think about it: Non-chan was in a hospital and Charlotte was implied to be in a hospital. Both had (or may have had, in Charlotte's case) cancer. And Non-chan is open for interpretation due to her limited screen time. At the same time, if she was Charlotte, then her familiars could've wandered off to Mitakihara and became another Charlotte and gained some new memories. After all, considering the time frame of both shows are at least a decade apart, it would be plausible.
Ojamajo 16 will be a Reconstruction if it becomes an anime.
Considering what Puella Magi Madoka Magica was, it wouldn't be surprising.