Break away fasteners are a thing now. Line it with some Kevlar fibre and some good thermal insulation/fire resistance and you have an amazing utility device.
In public, it billows behind you, making you look dashing and heroic. When the shit hits the fan, instant bullet resistant cover for civilians. A way to shield them from the heat of a fire, or a small explosion. You could even use it offensively to tangle or deceive an opponent!
Imagine if you suddenly get cornered by a runaway bull. What would you do without a cape?
That works but the fastener can’t be metal. Come across somebody with magnetic powers or for some reason the metal gets heated up and welds to itself… Bad times. But I think you’re onto something.
There is absolutely no way in hell a bullet-proof cape is billowing in the wind.
It won’t stop a direct shot, but it would help against ricochet and shrapnel.
Back during the Napoleonic wars, silk underlayers were highly sought after. They could limit the damage a musket ball could do.
A spider silk based cape could definitely help projectile damage, while still being able to billow. The challenge would be making it fire and heat proof as well.
