IMO, the better (and more radical) option would be to deconvert portions of the Kennedy running through West Loop & River West into a boulevard. Compared to just capping the expressway, deconversion would have the benefit of actually integrating the street into the urban fabric and opening up more land to development. This option is what Toronto is envisioning for the Gardiner expressway, here's how it could look:
https://www.cnu.org/highways-bouleva...cities/toronto