Make Hexdrinker/Maw shield start lower and scale harder with the user.
A simple nerf to contain the ADCs/assassins running rampant with these items.
Make it so the lifeline passive starts lower (150-ish) but scales harder with MR. Maw already includes this, but its base shield is still relatively high and the scaling is underwhelming (+1 to the shield value for each point of bonus MR).
What this would mean is that champions would need more than one MR item to reach the current efficiency of hexdrinker/maw. It would mean higher investment to deal with magic damage champions so it feels a bit more balanced going against AP or AD champions, rather than one-sided, especially someone has a small lead (defensive leads are much more apparent than offensive ones).