Allowing safe, uninterrupted movement of species. All ecosystems would be (almost) entirely connected to their neighbors, human settlement has upset this and its causing fragmented struggling ecosystems
The development in most of the areas in the eastern US is probably too far gone (and pipe dream projects like I dreamed up a few comments above are surely impossible), but I wonder some kind of special purpose multi-state consortium couldn't be arranged (and bolstered by relevant federal programs) to take something like my pipe dream 'southeastern mountains to the sea' and make a Georgia - Carolinas regional authority to fund things like buying out and cleaning up defunct industries in the region(s) and returning the land to a wilderness trust managed along the lines of the Adirondack Park Authority with the long term goal of creating natural corridors..
Corridors are a staple tool in ecology and environmental management.
They made the system much more resilient to local extinctions of key species and have a lot of extra benefits, like dealing with the excess of grazing by a big herbivore population that grows too much and is migrant by nature, or removing pressure in territorial animals.
Two places and a corridor act as one big place. Can host also new species that need to roam and never couldn't live in two isolated islands with the same total area.