The plans supposedly will allow at least part of the northern island and the fort to remain. So I don't think the plans are really anything like what you have depicted. Insiders have indicated that while there might be some reconfiguration and even loss of part of the river that it was largely remain as is and this "beyond Thunder Mountain" stuff would be put on undeveloped land on the north side of the current river.
@ToTBellHop just wrote a short bit ago that the plan is to continue the path that is around the Frontierland train station/Splash bathrooms and go north from there - I would presume they would build a bridge over the canal leading to the Seven Seas lagoon and then use the space outside the RR berm to build Coco and Encanto.