But, by passive-blocking do you mean that I wouldn't actively have to hold it up for it to actually block? Then, what's the point of even having it one me. It's not a shield, then. It's just a piece our armor, which would (and should) only passively block, when a certain part of our frame was hit. Like the fore-arm, if the shield was held in it, or a part of it, like a gauntlet.
It could transform, as you suggested. Turn from a gauntlet-type shield, to e.g.: a Shock Rail (I have been thinking about a design like that, for both EM-8ER and RWBY) or some other gun, then when you'd press block it would fold back and you could hold in front of you, otherwise how would it block, passively. Only when your forearm would be hit by bullets. Passive blocking sounds lazy to me and we'll probably have shields, as in an invisible field of energy around us, indicated by a blue-bar. That's as passive as it can get. If we're to have shields, they should have to be used, actively.
A counter-system should be limited, depending on the type of enemy we engage, or it would be heavily exploitable, if one could just counter the attack of every opponent. The only way it would be balanced, is if not all attacks of all type of enemies could be countered and not all of them could be stunned. But, again..I think all of us want too much detail in every aspect. When we talk about shields and melee, everyone is secretly thinking of Dark Souls or something, and wants something like bashing, countering, counter-stunning...etc.
We'll be happy if, after an initial single-swipe, we can have a 2 or 3 hit combo with our melee weapon and some will be happy if they can hold up a shield in front of them that they can modify to expand, so they could block whole groups or have it reflect shots or reflect the environment so it would act as a cloaking wall. Those are good uses. But, a counter-system should have only limited use.