You could still have grates that turn off.
Just make them part of a moving object and have them slide into a wall.
Personally, I don't really care much WHAT the thing looks like. What I'd prefer to have is CONSISTENCY - consistency from map to map, author to author - I don't want a forcefield in one map to allow me to shoot through it when I'm accustomed to not being able to and having to spend (half-)hours agonizing over what to do next. I don't mind having to learn things, but map authors aren't going to want to put a tutorial with each one of their maps on what the different surfaces do, and players aren't going to want to repeat the same basic tasks repeatedly.
The grate is pretty straightforward for a shoot-through-but-not-walk-through surface (B1) and so far it seems that the combine field (B2) is another solution identical in nature to the glass already in the game (also B2).
A1 (air) and A2 (fizzler) are already implemented, so I'm not quite sure why everyone is still discussing all of this.
If anything, we lack the ability to distinguish whether objects (cubes, cameras, spheres, chairs) go through these surfaces as well.
Let Y = "yes, objects pass through" and N = "no, objects do not pass through".
That gives us :
A1Y can walk through, can portal through (air)
A2Y can walk through, cannot portal through
B1Y cannot walk through, can portal through
B2Y cannot walk through, cannot portal through
A1N can walk through, can portal through
A2N can walk through, cannot portal through (fizzler)
B1N cannot walk through, can portal through (grate)
B2N cannot walk through, cannot portal through (glass, ff)
Perhaps the combine forcefield should be more typically used for one of the four categories not currently implemented? And I've seen two types used so far, a white one and a green one - which suits which better?
This is also assuming that you can see through all of the above 8 categories and that any wall you cannot see through you should not be able to portal/walk/pass objects through for the sake of sanity.