While I'm not personally a fan of Grievous Wounds, I would definitely support a GW item for AD assassins, so long as the game's current environment continues to be balanced around healing getting countered by healing reduction. Not only is the idea of Grievous Wounds itself probably the best fit for assassins, more than any other class (assassins typically set out to inflict grievous wounds upon their enemies, usually with bladed weaponry), assassins have also been struggling in a game state where their prime targets, i.e. marksmen, often have far too much access to healing, both from life steal and their support. Having some item build off of Serrated Dirk and Executioner's Calling (plus 1k gold, or maybe a Long Sword and 650 gold) into a 2.9k gold Lethality item could go a long way towards letting assassins counter defenses on their targets that can otherwise easily get out of hand.
As for mana sustain, I feel that might be appropriate on a separate item, as I don't feel there is an innate link between mana sustain and Grievous Wounds. Moreover, having one item for GW and another for mana would likely be better for assassins, as some, like Zed, are manaless and wouldn't like to have to waste gold on an effect they have no use for.
I agree that there's abuse potential on both of these items, though I think it primarily comes from the fact that Lethality is far too general-purpose a stat right now: it was intended for AD assassins to amplify their damage against targets with low armor, but the problem is that there is nothing really in an assassin's kit that makes them that much more disposed towards Lethality than, say, a marksman or even a physical damage tank. In fact, marksmen like Jhin and MF are particularly well-disposed to buy Lethality, because they're implicitly designed to apply burst, and also have the range to assassinate back liners from a distance. Tanks like Sion, who have very high base damages, get to amplify all of that damage with Lethality, no matter how bad their scalings are, which is why some abuse cases have cropped up recently. To fix this, I think eventually Riot will need to specifically work towards making Lethality good on assassins, but not other classes, whether it be via Lethality scalings on their kits or some other kind of roster-wide change.
