The last words of Augustus are commonly cited as:
Ācta est fābula. Plaudite!
The story is finished. Applaud!
Suetonius disagrees, and quotes a similar sentiment in Greek instead. But the objective historical accuracy of Suetonius is debatable too, so, leaving that aside…
This is a present perfective form in Latin, indicating that the action was performed, ended, and its aftereffects are now relevant (i.e. the audience is applauding in the present because the play finished in the past). It looks the same as the simple past tense, but acts like a present in some complicated grammatical ways.
We could quite easily negate this sentence, and add jam for "not yet":
Ācta non jam fābula.
The story is not yet finished.
But while this specifies that the story isn't finished, it doesn't specify that it's ongoing, either. It might never have been started in the first place. If someone is familiar with the phrase above, they'll recognize it as "the story isn't over yet", but someone who doesn't know the phrase could also translate it as "the story hasn't been told yet".
So another option is to change the verb:
Fābula non jam dēsiit.
The story has not ended yet.
With this phrasing the meaning is unmistakable, but the callback to Augustus isn't as clear. It's your choice which you prefer.