I think it would be possible to allow very basic, rudimentary, filters/effects similar to how flash operates. There already is raster and vector, so it wouldn't feel out of place.

Take this very simple shape I made in flash 8:

And a few visual examples of what they could look like:

Reason for this to be implemented, is say you want a bright white hot sun, or if you want a light to shine you could just use a glow effect. It be nice to do this in Pencil2D directly instead of having to boot into natron or after effects just for one thing.