While Ahsoka delivered a satisfying finale, anyone without a near-encyclopedic knowledge of The Clone Wars, Star Wars Rebels, and even the Expanded Universe may have found themselves lost.
It was in The Clone Wars that we first met The Ones - later known as the Mortis Gods - ancient beings who are thought to have been there when the Force first came into existence. The Father, who loves his children equally, represents the balance between the Light (The Daughter) and Dark (The Son). His children drew their power from the Font of Power and Pool of Knowledge, making them incredibly powerful, God-like creatures.
The Font is strong in the Dark Side and grants its user power over the past and the future; the Pool, on the other hand, is bathed in the Light Side and contains limitless knowledge.
As we've already explained, Obi-Wan Kenobi, Anakin Skywalker, and Ahsoka Tano eventually encountered The Ones, learning The Father hoped Anakin might take his place due to the Jedi's conflicted nature. Instead, The Son accidentally killed The Daughter, and The Father took his own life to rob his Son of immortality (allowing Anakin to kill him).
We're pretty positive that their stories have ended, though it's feasible that Filoni plans to use some of Ahsoka's leads as stand-ins for The Ones. Ahsoka, for example, still seems like an obvious choice to take the place of The Daughter. This may have even been his original plan for the former Jedi, hence why she showed up at the end of Star Wars Rebels looking so different before Ahsoka retconned her transformation.
Also of note is the fact The Ones have ties to the World Between Worlds and the Nightsisters, so their importance to the wider Galaxy can't be underestimated.
Their time is over, though, and more intriguing is what calls to Baylan Skoll on Peridea. We see The Father's gigantic statue pointing to a light in the distance, and while The Son also stands tall, The Daughter has crumbled away, a hint perhaps that Darkness is the predominant Force in this Galaxy.
As for who is calling out, we believe it's The Mother, a.k.a. Abeloth.
An Expanded Universe character, she was a servant to The Ones who eventually took on a mother-like role in the family. Despite initially keeping the warring children in check, she fell into temptation and her own quest for immortality saw her draw from both the Font and Pool.
As a result, she became wildly powerful and transformed into a monstrous and near-unstoppable Dark Side being called Abeloth. The Ones, horrified by her betrayal and what The Mother had turned into, imprisoned her within a planet and left to take refuge on, you guessed it, Mortis.
She remained safely imprisoned until they died and, with her cage weakened, Abeloth was ultimately freed by the son of Han Solo and Princess Leia, Jacen Solo (not to be confused with Jacen Syndulla). She wreaked havoc across the Galaxy as an even greater threat than Emperor Palpatine but was eventually stopped by Luke Skywalker.
It's unclear how Grand Admiral Thrawn, the rise of the First Order, and Supreme Leader Snoke's emergence will tie into this (if indeed they do) but all signs point to Abeloth becoming the focal point of Star Wars storytelling in the years ahead.
Now, Filoni just needs to make it make sense for casual fans...