Mana sustain? Perhaps. Siphoning of energy to gain some sort of power up? Maybe. From the energy flowing into her hands and her nature as a succubus who feeds upon ... something ... pain? lifeforce? Schlong? It would make sense to me that that feeding and siphoning theme be present in her gameplay. However I think anything that makes her more survivable is out. She's supposed to be an accessable assassin who can 100 - 0 people while also being able to move much more freely without giving enemies time to react thanks to her ability to perma stealth. I think those capabilities alone, along with the assumption she maintains a speed buff / gap close, and or hard CC in the form of a charm, makes me worry just how much they'd have to gate what she's good at just to give her sustain. Currently she has a mana pool that can go down fairly quickly because of her Q spam in continued engages, but regenerates faster while in stealth as a bonus to her passive. If anything I think they'd have her gain mana when she gets a kill or based on damage to champs or something if they decided to incorporate that theme.