Well, we were talking about this on the brawl+ chat, and it's a pretty solid idea. When you have upward momentum you're acted on by normal gravity, so recoveries and upward kill moves aren't nerfed, but when you're moving downward, you have a higher gravity value acting upon you (I don't think we want quite 1.5x gravity... maybe 1.3x or so), so characters can't simply float back to the stage after getting hit out, and actually would have to use their upBs, which drastically helps the edgeguarding game, while also allowing characters to reach the ground quicker while comboing. It's a pretty solid idea overall, though how easy it will be to code and what odd effects it will have on gameplay when the grav switches is still uncertain.
There's also the problem that this game needs running momentum to transfer into air momentum... it's so stupid the way it is now >_>