Zenon: Hey man. You make a comment that lifesteal / healing tops you up but you remain fragile. Thats what you need to hit on.
You need to retune all healing.
Whats wrong now is that all healing is inherently more effective on higher resistance targets. Mundo not only heals 60% of max HP, but builds stupidly high resistances.
Mundo is a problem.
I can build 40% lifesteal on an ADC and still get gibbed in 2s by an assassin.
ADC sustain is not a problem.
So, I have a very simple solution: Have lifesteal and heal effects heal you for that amount of effective HP.
Say you have 200 armour and 100 MR: That means you take 33% of any incoming physical damage and 50% of any magical damage. You're a I dunno, shen, and on 300 of 2000 hp. Now, Your total effective HP is 6000 vs physical, and 4000 vs magical, for an average of 5000.
Now, your soraka Wishes back 500 HP. You have an effective HP-> Hp ratio of 2.5 You only heal for 200 real HP.
Your ADC teammate who has exactly zero mr and armour due to {reasons} heals for the full 500.
Boom. Now, we have healing is inherently less effective on tankier targets. This means that when you apply a heal of X, damage of X will exactly negate it.
In order to keep some champions balanced (mundo, garen) we change their passives to heal a percent of their effective max HP per second.
So, lets see what we've got for GW now. Well, tough champions inherently heal worse than fragile ones, so we can just redamage them rather than having our only option be GW away the heal. As for squishy champions, we can burst them.
There we go, we have no reason to have GW any more.
Lets look at the three main classes of healers: ADC's and fragile drain tanks (fid, swain). You can burst these champions 100->0. Or, you can grind them down. Basically, the fight will go as normal, but at the end of it, these champions will quickly hit 100% Hp again. No GW needed.
Next are tough drain tanks. Shyv, Xin, Trundle: Through lifesteal or other reasons, these champions no longer exploit healing more effectively than more fragile champions, since healing for X is wiped out with X damage. There's no need to inhibit their healing now.
Finally, there are champions with heal casts: These are now automatically less effective when cast on a tough teammate, meaning that you can't just top up a shen or something with some cheap heal requiring a massive damage spike to deal with. These heals can be tuned for use on ADC's / Mages, because even at full HP those champions can be spiked.
There you go Zenon.