Another headcanon:
Despite Weiss being the most powerful of the Tsviets, Scarlet was always the most proud of Rosso. The two shared a nigh sexual lust for bloodshed that made Scarlet look upon Rosso as a daughter or younger sister, formed in the image of Scarlet's innermost desires.
If Scarlet had ended up in a scenario where Rosso was about to kill her, Scarlet could have saved herself some time by offering to hand over even more efficient and satisfying weapons for mass slaughter.
EDIT: In fact I think that a flashback scene between Rosso and Scarlet would help make Rosso a more interesting character.
Headcanon #2:
The dome where the Deepground city lies is a Spiran settlement. 2000 years prior to the game, Spirans, using some their still functioning machines, dug deep into the earth and settled underground to hide from Jenova. The Spiran tech is what's behind the city-encompassing holograms, the air being breathable etc. However the batteries they used ran out of power and so it was abandoned. When Shinra later discovered the underground dome, he had the Spiran machinery activated by hooking it to Mako. So the explanation that could make Deepground more believable is, essentially...
I also imagine that some machinery, like the elevator (where you fight Azul) that leads down to the city is of Spiran design. Same with the "suspended railway" Vincent use that takes him through the city.
Before getting to Azul, you can collect five "Top Secret Files". I have a fantasy where, in a rerelease of Dirge of Cerberus, these are readable files. These would tell how the underground dome was found, how the machinery was activated, and how speculations about the place being made by Cetrans quickly moved to speculations about "
Others" being responsible due to the place being way too hi-tech to be of Cetran origin.
