As I mentioned above, we're building a new app for client communication on a brand new stack at ProjectPulse. Since Bootstrap v4 is stuck in alpha still, I decided to look for other options.
Option A. Foundation - I think what ZURB does with Foundation is excellent, but they build towards Compass and Angular, neither of which I use at ProjectPulse.
Option B. Semantic UI - more full-featured, especially with modules, than Bootstrap ever has been. This allowed me to cut down the initial front-end stack by 5 libraries on the new app. Winner!