Hi Lester,
First of all,
Nick Risinger's Photopic Sky Survey is one of my favourite sources for deep wide field images free from light pollution gradients, etc.
As for your long faint line... with some drastic stretching it looks like that long dark line is present in the Sky Survey photos (pic #1). I dug up one of my
recent deep wide field exposures and found the same thing even stronger (pic #2). Both pics are inverted RGB and rotated to have a similar orientation to the one you posted.
The line appears to pass through the Reticulum constellation so I had a look at the region in WikiSky's infrared data... and ta-da! (see pic #3). It looks like it might be some sort of dark nebula?
I remember that there were a few posts in Deep Space a couple of years ago trying to image the bridge between the LMC/SMC and the Milky Way - with some links to really, really deep exposures. They may be worth a look too?