Dosbox already has a large variety of scalars built in. If there is a particular scalar you know to be superior, you could suggest it, but I think you are asking for something that seems easy to you and is incredibly hard for a computer.
Scalar shaders work by examining the frame buffer. It is VERY hard by looking at the frame buffer for a scalar to determine the difference between one object and the next. Asking a shader to "smooth out this sprite" is nearly impossible, given the sprite could have many colors, be any arbitrary shape, could have transparency, etc etc.
Believe me considerable work and thought has gone into the scalars that are already in Dosbox, and it's not just dosbox, they are shared by a huge variety of emulators across many platforms. People have been trying for more than a decade of emulation to come up with the "right" shader, and so far no luck. Different ones are strong in different circumstances, so I would just play around with options available.
I personally dont want a smart shader at all. I prefer the graphics to look pretty much as they did in the original game without modification, so I usually pick a normal2x or normal3x for instance.
-Frob