There is a new 50 story mixed-use development with 800 residential units, ~12,000sf retail, and 399 parking spaces planned at 20 Montgomery. It would replace a parking lot next to the Harborside complex. This developer is also referred to as HARBORSIDE PLAZA IV - LOT 29, BLOCK 11603.
No renderings as of yet, but I'm sure one will be posted soon.
This is an older proposal for an office building at the site, at 38 floors.