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This particular change may have already been suggested, but since there's been a lot of discussion concerning this item I thought I might chime in.
What if you make Executioner's Calling reduce healing done instead of received? This will help it not completely negate healing spells in team fights unless you focus on the healer for at least 1 hit, as well as not getting rid of its 1v1/ganking potency.
This makes it so healing abilities in general require some effort to counter instead of just being completely ignored.
This of course does not fix the fact that this item completely negates Mundo's ultimate, which I believe should be change in some way. Maybe make his ultimate immune to it, but there's another thread dedicated to that specifically.