Hmm.

You could go simple, like just resizing to a lower resolution then upscaling it.

You could use avisynth's Blur() (which is gaussian and the reverse of avisynth's Sharpen().)

You could also use fft3dfilter, but that's pretty CPU intensive just to mutilate an image.

Once you've softened things, you may want to use something like addgrain() to dirty it up.

I can't think of anything else you'd really want to do. I can't see you wanting interlace artifacts or image jitter, but I suppose you could.