>>530898989
I imagine the biggest threat to human Ciel, would be old age - she's human, and the term 'centuries' is thrown around often in regards to the Game of Destiny, and by multiple characters, characters that would have no reason to exaggerate length of time. See, you can point at one character saying it, and go 'oh, it's flowery to emphasize their suffering', but when another backs it up, it's consistency.

Of course, with the developments in cybernetic technology, humans can live a long time - going back to Weil, his suit was made to sustain him biologically, and his memories were digitized to prevent him from mentally degrading, so the technology IS there, and has probably gotten better over the centuries (Compact, discrete).
However, I don't know if Ciel would subject herself to Legion's body modification program, because Albert was deeply involved in that, and he would probably have records on her, messed with her genetically, put a tracking device in her, etc - Albert was paranoid, and I don't think it would be in his character, to NOT keep an eye on Ciel, considering she had a hand dealing with Weil, and she's got an intelligence that would make her a threat. However, she only ended up acting in opposition to him very late into the game, when Serpent started causing issues, and I can't imagine Albert PERSONALLY handled every procedure, so maybe she got it early on when Albert wasn't very diligent/underestimated her and didn't take precautions, etc.
If not legion modifications, though, then question is: could she do it herself? Modifying wetware and implanting augments is a lot different from developing a reploid, so it's not in her inherent feats. Is Ciel smart enough to learn human augmentation? Probably? Especially with the timeframe - she could have developed reploids, or automated tech to do it for her, to circumvent the feds.

So, a human Ciel in ZXC would be incredibly old, and probably need some crazy tech to keep herself going.