With resepct to Nikki coming back, given that:
1 - Obscure Death: Nikki's death is not shown on screen and is therefore, not certain; and,
2- Resurrection Power: i.e., the power to bring somebody back from the dead is a real possibility in the Heroes milieu; and,
3- Flashbacks: Flashback scenes in which Nikki is involved do not require any alteration to any death story; and,
4- Illusionary Participant: The power of illusion and shapeshifting present in the Heroes series is a real force and allows for limited involvement of "Nikki" (or a simulacrum thereof) in a future storyline; and,
5- Time Travel: As distinct from a flashback, the presence of time travel on the Heroes series as a real theme and typical plot device leads us to yet another possible way to involve a dead character in any current storyline,
We can comfortably say with certainty that nothing is certain regarding Nikki. She may be back at some point in the future - she may not be. We simpy do not know.