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.
Doremi is related to Tsubomi Hanasaki.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.
Fami will become a Kamen Rider when she grows up.Months after returning to her own time after the encounter with Doremi, Fami, after going through all the witch tests, decides to give up her witchhood and move on with her normal life. Then, a few years later, she survives a solar eclipse ritual and winds up becoming a female Kamen Rider with powers similar to Wizard's, possibly a Kamen Rider Mage. Afterwards, she winds up meeting and teaming up with Kotaro/New Den-O and Miharu/Kamen Rider Aqua. Whether this will increase her chances of dying or not is still up in the air, but given her bad luck it's quite likely.
Majo Kai was at one point a Witch's barrier.The Incubators have been helping humanity since prehistoric times and it's never been established how long the witches from Ojamajo Doremi have existed. In all likelihood, Majo Kai could have been the barrier of an ancient Witch that was later commandeered by the ancestors of today's witches (or these witches could be an evolution of the Witch's Familiars). In fact, the magic of Ojamajo Doremi could very well have been descended from the magic of Puella Magi Madoka Magica!
Kotake will become Fami's grandfather.