Semira isn't actually deadSemira is stabbed through the back of the head which apparently kills her. However, at this point, she was a Vampire-Corvinus Strain Hybrid who have been shown to have equal, or even superior, power and strength to the Elders. Previously, Viktor survived virtually the same injury; therefore, unless they torched her body then Semira surely could've survived such an injury as well.
Semira was Viktor's protégee.But he sent her away in favor of Selene, due to her resemblance of his daughter. Possibly in his last moment, Viktor was thinking of Semira.
- Semira herself has a slight resemblance to Sonja (they're both pale and dark-haired), Semira was said to have been his regent and clearly held Viktor in a high regard. She also appears to be jealous of Selene and is resentful over her killing him. It would therefore be plausible that Viktor regarded her as a protégee of sorts, but ditched her in favour of Selene.
Michael isn't actually deadThe only source to confirm that Michael is dead is Marius's blood memories, and it may be that Selene only saw part of those memories, or Marius was unaware of the scale of Michael's regenerative abilities and he would eventually recover from the implied bloodloss unless they actively destroyed his body, which they would have had no reason to do.
Lucian isn't Dead.Multiple people have noted that Raze dropped the syringes containing Amelia's blood, near Lucian, who then woke up, he had already injected himself with Michaels blood, like Michael, grievous injuries take a good while to heal, so Lucian woke up as a hybrid after the events of the movie, but with Viktor dead, his best friend dead and the only Elder left still asleep and having no real issue with Lycans he decided to get out of dodge, and may turn up if there is any other sequels, perhaps maybe even to kindle something with a near identical to Sonja Selene.
Sonja and Lucians child would have been the only "True" Hybrid in the franchise, and more powerful than anything seen.All of the Hybrids thus far have been "uneven" Hybrids, None except Eve occurred Naturally, and she was even more uneven, with everything being gained through two "Turned" uneven Hybrids, But Sonja and Lucien's Child was the product of two "Pureblood" Members of the Species, Viktors Bloodline, which is the second oldest Vampire in existence and Lucian, the very first Lycan, Their Hybrid Child would have been an "even" Hybrid, meaning their child would have pure Lycan and Vampire Hardcoded DNA, rather than the later Hybrids being turned into Hybrids, this would make them much more powerful than any of them, because they would grow into their power rather than have it suddenly given to them.
Michael & Marcus were never as powerful as they thought they were going to be.Related to the above, Both Michael and Marcus were "uneven" Hybrids, due to how they got their Hybrid status, Michael was much weaker than he should have been, because he was turned, not born, and also because of Selene, she was neither born a Vampire, nor an Elder, if Michael had been given an injection of Amelias blood, he would have been much more powerful, The same goes for Marcus, while he is the first Vampire, he isn't a born vampire, also the blood he got to turn into a Hybrid was from a Turned Lycan, if he had gotten blood from Lucian or another born Lycan he would have been much more powerful, there "uneven" status is even shown by their transformed state, Michael is clearly more Lycan than Vampire, and Marcus is more Vampiric than Lycan, in fact, Marcus exhibits very little Lycan traits, barring the ability to transform.
The "plague" that wiped out Alexanders home and started everything was actually supernatural.Just due to the impossibility of many of the abilities of the Immortals in Underworld, Memories aren't kept in blood, the rapid transformation of the Lycans, that weird Flash Step The Elfy Vampires can do, not to mention nothing in nature is able to be Truly "immortal" like they are, the plague was a supernatural attack of which Alexander was the only survivor, due to a quirk in his genes, perhaps a latent affinity for magic, but being a 5th century warlord kept faith, hence why he never tried to exploit any other abilities he may have gained for the "plague".
Viktor will somehow be Back from the Dead, attempting to retake control over the vampires again.