I don’t see this as broken. There’s a few niche shields in the game which have this already.
The alternative is if we can add some new MI’s that I can hunt
E.g.
Give Obsidian Bulwark’s %Physical to Fire
Give Fleshwarped Bulwark %Physical to Aether
Give Spectral War Shield %Physical to Vitality etc etc
This is an issue that become apparent when i wanted to do a shield caster but there’s no shield that converts damage for the damage type i was gonna use.