The area in question is circled in red with a red arrow pointed to it, right around Ilex Forest.
Granted, the map is apparently a rendition of HGSS, not GSC, but it has me extremely curious. Normally, I would pass it off as a new addition, but I've seen it before, in a dream of all places.
When I was really into Crystal, around 2002, I had a dream of a "secret area" next to Ilex Forest that involved two lakes. You had to activate the shrine to enter it, and you could catch Entei and Raikou just as easily as you could catch Suicine in the Tin Tower.
I didn't think much of it, because I knew it was a dream, but now this picture looks extremely similar to the place in my dream. Was it ever a part of GSC? Was it some sort of special area that you could only get by attending a Nintendo event and downloading/adding it in? Maybe this is the wrong place to ask, since it's probably just a new addition to HGSS, but I can't get over the fact that I "saw" this place before this picture was even drawn. Maybe I'm just insane...
Anyway, thanks for all help!