What if shields could be penetrated from behind?

It would give assassins the strategic niche of being capable of flanking and using their mobility to burst squishies from behind, who are too busy focusing the frontline of a teamfight whilst being kept constantly full hp by their enchanter/anyone with locket/lifesteal. It would finally allow a difference between assassins and divers rather than, "Oh, assassins have a way out while divers don't" (which isn't always true, particularly if you needed to use your mobility to get in, which against smart teams is necessary).

Plus, we finally have a way to out-play that Any ADC/Enchanter combo rather than hoping one of them does something stupid or gets caught out by themselves.

For tanks who rely on self-shields, they could instead receive an alternate shield, a 'barrier' that fully encircles them (or, leave their shields as forward-blocking-only, and allow fighters to use their mobility to outplay these shields by getting behind).

It won't solve all the issues, but it'll alleviate a lot of those 'impenetrable fortress' type of comps and give selective viability to the assassin role, who really is just a discount mage with slightly stronger snowball potential, but a /currently/ irrelevant late game.