In Synfig and Krita there is the concept of a "filter layer", i.e. a special kind of layer that affects all layers below it or in the same group of layers. Would it be possible to implement this in Inkscape? Or does it exist, in some way or another?

Description:
For instance, in a drawing with Layers 1, 2 and 3, add a new layer "filter layer" on top of layers 1 and 2 but below layer 3. This "filter layer" has no objects, but has properties (filter parameters) that can be set (same as custom filter, with presets like blur, opacity, and complex combinations of parameters). All objects on any layer below this "filter layer" will have the filter applied. Objects above remain untouched.

Added the possibility to use a mask on the filter layer (a "hole" in it where it doesn't apply), it's quite handy...

Svg specs doesn't have a support for layers; inkscape implements them as groups.

You can use layers and sublayers if you will, so that a filter added to the layer would affect its sublayers.
In your example, a filtered layer with sublayers 1 and 2, and a separate layer 3 could function similar as you described.
But filters doesn't work by "affecting objects below".

vote for bug https://bugs.launchpad.net/inkscape/+bug/1201610

v1nce wrote:vote for bug https://bugs.launchpad.net/inkscape/+bug/1201610

I already did

So, if I understand correctly, feature request #1201610 together with use of sublayers would do the trick. (Maybe mention also that feature request #512834 is more or less a duplicate, and gives a rather easy workaround. So it is already possible, actually, only not directly through dialogs).

Would it be possible to "make a hole" in the area where the filter has effect (like applying a mask)?