It's simple, really.
A shield meter.
I mean, currently, we can try to activate our shield to see how much more we can take but that gets to be a pain. It'd make life a lot easier if we could see a tiny little bar, gauge, circle, or something that showed what stage/color our shield was/is currently at.
That's all.
It doesn't necessarily have to be a "bar" persay. Just some way to gauge your current shield.
Heck, it could be something as simple as a colored dot that follows your shield status. You get the same effect, being a method to tell how much your shield has left without actually having to use your shield at a (possibly) inconvenient time.
We have a bar for our health, I don't see why we can't have something for the shield. As it stands now, I never know when my shield can take that "second" hit, often leading to a hit that I wasn't expecting, and far more frequently, a status effect I might've avoided if I tried to run instead of shield.
Perhaps a glow around your equipped physical shield? That would be a straightforward way to fit it into existing graphics without adding odd little floating status indicators.
I'm against a actual bar, much for the same reason I dislike all the "weapons need numbers!" chat.
(not going into that, suffice to say I don't like there being a 'best' piece of armor or equipment)
I would like some sort of flash when your shield is full recharged tho.