To follow on from sidd's comment, a fully integrated North East US + Eastern Canada grid would seem to provide the best option at first glance. Quebec Hydro provides the big hydro battery, with a great mix and diversity of possible sites for wind and solar across the North East US and Eastern Canada.
Same could be true for Western Canada and the Northwestern U.S.
Yes. But not just the US Northwest. Everything west of the Rockies needs to be tied together. Between Western Canada hydro, PNW hydro and wind, Idaho hydro, Wyoming wind, Southwest solar, Nevada (and potentially SE Utah) geothermal there's a lot of generation variety that could create a large grid with only modest needs for storage.
The backbone of that large grid is already in place. There's a HVDC line running from LA to the PNW (Pacific Intertie) which mainly brings hydro to SoCal but can, as well, take SoCal/SW solar north. There's also a HVDC line running from LA to Utah (Mountain Intertie) which was built to carry coal generation to SoCal. That is being freed up.
Plans have been drawn to connect the north ends of the Pacific and Mountain Interties in order to create a transmission loop which increases reliability. And then add a spur into Wyoming to pick up the very excellent wind available there. Companies are looking at adding pump-up hydro storage in Utah which could then serve the entire grid. Cheap land with lots of abrupt elevation changes.
The Pacific Intertie runs close to where most of the Nevada geothermal is located. And there is at least one lower voltage line running to the NorCal coast where some excellent offshore wind resources are located. The land is secured. It would just mean increasing the size/voltage of the line.
Then there's a spot in New Mexico where the three main US grids (Western, ERCOT, and Eastern) come with about 60 miles of each other. If Texas/ERCOT would cooperate we could tie all of the US lower 48, lots of Canada and Northern Mexico together and share power over a very large area.