Overheal, my concerns regarding this rune.
Overheal is a good idea on paper, sometimes you use your Healing ability, someone else heals you ect. And you don't get the full benefit's because your already pretty close to full health.
The issue i have with this though is pretty simple, if this applies with life steal and stacks with Bloodthirster that's going to be a balance nightmare. Lifesteal isn't bought just by ADC's, many fighter's buy it as well. Fiora, Riven, Renekton, the list goes on. In many cases these champions are hard enough to deal with without an extra 10% maximum health sheild, Illaoi doesn't build much life steal, usually settling for a Death's Dance and Black Clever, but with Overheal she might become even more threatening, her and many other Juggernauts such as Darius, should not have a passive shield. ADC's should either, their damage output is higher than any other champion in the game and when they get lifesteal they can have anywhere from 2000-3000+ effective health. Giving them a shield i feel would throw them even further out of balance, assassins have a hard enough time taking down an ADC once 30 minute's hit's and they get their 2-3 items, give them an extra shield and they might actually become unkillable in the right circumstances.
Even now, i was playing Katarina, pretty fed 10/4/6 about 32 minutes in the game, enemy ADC is a caitlynn, pretty easy to one shot usually, so i went to go kill her as she was farming bot lane, i failed miserably. 4 item kat < three item Caitlynn. Let me explain, basically i approached her from her blind spot, flanked her, i went in like a normal kat does, grievous wounds on my R and all, but the thing is the whole time i'm approaching her she's auto attacking me, dealing 200-500 damage per auto with the I.E, Runaans and Headshot's, healing with bloodthirster, gaining a shield, i got her to 100 health twice that fight. Meaning her 1600 health was gone, twice but because she had life steal and heal, i basically had to deal with someone who did 1500 damage to me in the space of 2 seconds and had 3200 health, unlike her i don't heal for my entire health bar with one spell or auto, so i was basically going to lose no matter what.
And i can't tell you how many times that's happened to me when playing as an assassin, i wish they'd give new items to us or a new Rune with the new rune system to help assassins out because ADC's are honestly too tanky with how much healing and shielding they get, if her support was there she'd have had at least 4000 effective health with Redemption+Locket and if it was a soraka, she would have atleast 5000. You expect an assassin to do 5000 damage in the span of 2 seconds, because 2 seconds is about how long they'll live against an ADC who keeps criting them to death.
TLDR: I don't think Overheal is needed, or at the least it should only work with Healing abilities, not summoner spells or life steal.