A Lifesteal Passive Concept

So I was looking through champions earlier and I started getting pretty interested in their passives - for some champions they were just a bit of a buff here and there, like Cho'Gath's or Ahri's. For other champions, they were significant enough to change the playstyle of the champion or how their items were built, such as Akali or Rammus.

I was wondering - if a new champion were to be released with the passive along the lines of "Lifesteal applies to abilities", what would you think of it? The balance would heavily rely on their kit, of course, so what kind of abilities do you think would work well with this passive to make it potent, but not overpowered? Or could the passive be changed to mirror items like hextech gunblade and be 33% as effective for aoe attacks (even if the kit doesn't include aoe, items might), so that the kit they have could be more flexible without becoming op?

I could pump out some ideas here and now, and would be willing to, but I'm still fairly inexperienced in the game and would probably just make the entire thing even more broken than Senna and Aphelios's roid baby. I was more interested to see what you competent people might think of this or could bring to the table - forms of life drain have always been my favorite abilities in games.