Suspiriorum Returns To Power
This ending can completely reframe Suspiria and Susie's arc as a whole, but Guadagnino's execution doesn't provide any easy answers. We see in flashbacks and dream sequences that Susie was not raised in the ideal scenario, but her path appears to have been set from day one. Suspiria occasionally cuts back to a woman on her death bed who is revealed to be Susie's mother. The extremely religious parent believes Susie is a great sin she's unleashed upon the planet, and says as much to the camera. Based on this belief, Suspiria appears to be communicating that Susie has been the new Suspiriorum for her entire life. The question that then arises is whether or not Susie has known this the entire time. Since it isn't explicitly made clear in the film, lets explore both options.
Susie knowing her true destiny would explain why she was the only one willing to volunteer to be the lead of Volk, because she understands the importance of it in the grander scheme of things. Her arrival at Marko Dance Academy coming at the perfect time could be explained as a god-like figure being omniscient, so she'd know a new lead was needed. It also comes as Susie is in the prime of her life, so Suspiriorum arguably could've waited until this time so that her reign can begin when she would theoretically be the most powerful - that is, if we assume the academy has been attempting this ritual for several years, as possibly indicated by their several prisoners. This would explain why Susie isn't as disturbed by the nightmares forced upon her by the instructors and her understanding of the academy's larger goals. But, this line of thinking also means that Susie's obsession with the academy and training to be a dancer was all out of necessity for her own personal gain.
The other possibility is that Susie begins to realize her true identity over time. When she initially arrives in Berlin, she does not know where the academy is located and is star struck when Blanc steps in to watch her audition. She simply starts out trying to achieve her own personal goal of leading Volk, something she's studied for and dreamed of for years. If the academy building and Berlin are Suspiriorum's place of power, then it could also be argued that Susie's arrival begins to unlock the other side to her. In this scenario, the more time she spends at Marko Dance Academy and trains for the ritual, the more aware she becomes. This would still explain why she is accepting of the weirder aspects to the academy so quickly. This argument would postulate that Susie was destined to come to Berlin and was instinctively drawn to the Marko Dance Academy to fulfill her destiny and reclaim her place of power.
The Real Meaning Of Suspiria's Ending
Sometimes a look to the larger theme of a movie could help answer questions like the reason behind Suspiriorum's rise, but that may not be the case with Suspiria. One of the film's largest themes is power and the abuse of it. This can be seen most prominently in how the instructors run Marko Dance Academy and how Markos specifically uses her position of power to benefit her own gain. When Suspiriorum reveals herself, she eliminates anyone associated with the abused power within the academy. This gives are a clean slate to start over and use her newfound power as she deems fit.
Whether or not Suspiriorum will actually use her power correctly remains to be seen. After going this far to eradicate the academy of all power hungry and corrupt individuals, a new reign is upon us with Suspiriorum's return. There's potential for Guadagnino to address this if any sequels are made, but Susie does appear to be at least partially interested in self-preservation at this time. Killing Markos for falsely claiming to be Suspiriorum could be an understandable outcome, but immediately murdering her supporters without first seeing if they'd follow her lead could indicate Suspiriorum is just as abusive with her power as anyone else. It doesn't stop there either, as wiping Dr. Josef's mind without his consent is a step too far, and then there's the ominous after-credits scene. She may show some sense of mercy with her dealings with Patricia, Olga, and Sara, but the potential for Suspiriorum to abuse the power she is attempting to eliminate is there.
An abuse of power isn't all that Suspiria addresses, with Guadagnino touching on a range of different themes throughout, but it certainly is the most prevalent. Its a fitting end to the story he is telling that leaves the door open for discussion about how the theme of power relates to motherhood, feminism, and more. Suspiria wants to show that the abuse of power is wrong, but also conveys just how easy it is to abuse the power when trying to fix it.