Here's an idea:
BTMR is the only attraction that has to remain "The Old West." So... update the Rivers of America to really be the rivers of America like it is in Disneyland. You start at a Philadelphia port on the Delaware (Liberty Square Port). Then you go up the Mississippi incorporating New Orleans and The South (Bears and Tiana and Tom Sawyer). Then you round the bend to the Colorado River where BTMR is. Then up the Missouri and/or Yellowstone and/or Columbia/Snake river for Indian scenes. Then round the bend down the Hudson for Haunted Mansion.
This way, BTMR is part of the Rivers of America attraction rather than part of a Frontier Land. And, in the same way, Haunted Mansion is part of the Rivers of America and not part of Liberty Square.
Then we do away with "Frontierland" and replace it with "Mississippi Valley."