I would say one that works through USB but that's pretty damn slim considering such a thing wasn't done with the PSP.
At this point we can only speculate, but it seems pretty hopeless that we would be able to soft mod the system without some sort of exploit. All in all, we would need a game with the exploit for any soft mod in the first place, or even better. A flashcart that utilizes the exploit itself and can bypass the Nintendo protection. Sad yes, but better than no hack at all in my opinion. Perhaps someday, a soft mod will be created that could be used through the SD...though I honestly think one would need a flashcart with the exploit in some way(or a better exploit somewhere down the line)that would work in tandem with a program from the SD card.
That's as close to soft modding the 3DS will get I think.