I think it would be interesting to see a minification filter that averaged all of the texels that fall inside of a pixel on the screen. The end result would be somewhat like anistropic mip-mapping, except more accurate, and the user wouldn't have to construct all the mip-map levels.

I know it would be a lot slower than normal mip-mapping, and it would really cause problems if an entire texture mapped onto one pixel on the screen, but the quality would be extremely good, and maybe in the future graphics cards will be fast enough to do this.