After Ella heroically disenchants herself, the ending practically writes itself.
She and Char are married within the month, and her stepfamily is most definitely not invited. However, all their other friends and allies are: Ella renews her friendship with Areida, for instance, and they remain lifelong friends. The fairy Lucinda, who'd "blessed" Ella with obedience at birth, also shows up (uninvited), but instead of bestowing some heinous gift on them, gives them a small enchanted box that's a cute and harmless example of fairy magic.
Mandy comes and lives at the palace with Ella and Char, and she becomes fairy godmother to their children. Ella doesn't have much contact with her father, Dame Olga, Hattie, or Olive, but we're told that Olive marries a chatty widower who gives her cake, money, and conversation on a daily basis.
Rather than accept the title of princess, Ella becomes Cook's Helper and Court Linguist. She travels with Char when he must attend to royal business, and she picks up all the languages they encounter. Her fairy-made book helps her keep tabs on the kids when they're traveling. The marriage + kids + royalty equation is pretty standard for fairy-tale happy endings, but Ella has her happy ending on her own terms: she still has her own friends and identity, and she revels in being contrary since nobody can order her around anymore.
You know all this sounds like to us? It sounds a lot like being a grown-up. By learning to stand up for herself and—this is crucial—to act out of the right impulses, Ella has earned the right to make her own decisions. It's not just something that you get when you turn a certain age. It's like a driver's license: you have show that you deserve it, first.
And Ella sure deserves it. In the epilogue, we get to see Ella being completely normal: no longer bound to be obedient, just like any other human being not under a curse. We've been rooting for her all along, of course, but now we know that she finally has her heart's desire—freedom. If she'd wound up still subservient for whatever reason, the novel might've gotten tossed into a corner because seriously? Who wants to read a book where the main character both begins and ends in captivity?
So, keep that in mind if you ever write a novel.