I've been working on the "in game" GUI for the actual emulator (yes I know) and while I was there I decided to re-enable some old code I had, and that is shader based graphics filtering.
Now the difference between using Scale2x, HQ2X, etc and a GPU based approach is that you shift the burden from the CPU to the GPU. RetroCopy will allow you to use both at the same time however, as people like myself have enough power to run pretty much anything.
I'm not sure on what the final combinations will be by release time, however here is what there is to play with at the moment.
CPU Filters: Normal, Doubled, Tripled, Quadrupled, Scale2x, Scale4x
GPU Filters: Normal, Sai2X . And they can both be combined with either horizontal, vertical or grid based "scanlines", which are highly configurable to be as dark/light and spaced how you want.
The grid scanline effect can produce some interest results :-
Action Fighter in particular gets a big boost in the amount of visible colors, for a system that can only usually display 32 colors at once it seems to greatly improve visual quality.
The good thing about GLSL shader based filtering is you can have many different options for particular effects and you aren't as limited in regards to what you can do.