Once Empire between Rogers and Flatbush is rezoned to allow residential and greater density, I'd put a Trader Joes on the ground floor of an apartment building that I would build on the Western Beef site.
Alternatively, I'd consider the ground floor of the existing Bond Bakery building on Flatbush and Empire.
The spaces I just mentioned are likely beyond the means of the Park Slope Coop, which is reportedly considering opening a second location.
That rumor has been investigated a few times.
However, all indications are that the site if the former bank will become a 13 story condo building.