The idea is a screen fade (using register $2100) that would start with a fully brightened screen and the top and bottom of the screen would darken towards the center in a gradient fashion.
Think of someone blinking their eye, how the blackness comes together until you can see nothing. The nuance would be that you'd have different levels of darkness in the transition. For example, 16 frames into the sequence you'd have scanline 0 as 0 brightness (fully dark), scanline 1 would be brightness 1 (almost completely dark) all the way up to scanline 15 which would still be fully illuminated. The behavior at the bottom of the screen would be a mirror image of this. Most fades I've seen involved doing shapes with windowing (like the spotlight in Super Mario World) or just gradual complete screen fades (Super Metroid when going to the pause menu/map).

I have a pretty good idea of a couple methods to approach this, so I don't think I need any help with it, I'm just curious to know if anyone else has seen this sort of fade on the SNES before, as I like trying to do things that haven't been attempted (at least to my knowledge). For the curious my estimates show that using direct hdma tables that are completely precalculated would take up 0x2180 bytes (approximately). That would save on runtime computation, but I could also calculate the tables each frame pretty easily too. So... thoughts?