We sense we want to have more than the 3 breakpoints the Standards supply and thus a revised grid of our own. This is awaiting discussion with dev next week.
Possible breakpoints (I've bolded the breakpoints of the U.S. Web Standards):
Smartphone
320px - iPhone4/5 portrait, Galaxy S3/4 portrait
360px - Galaxy S5 portrait
375px - iPhone 6 portrait
414px - iPhone 6+ portrait
480px - iPhone 4 landscape
568px - iPhone 5 landscape
600px
640px - Galaxy S3/4/5 landscape
667px - iPhone 6 landscape
736px - iPhone 6+ landscape
Tablet, Laptop, Desktop
768px - iPad portrait
800px
1024px - Tablet landscape
1041px
1167px
1200px
1224px
1280px
1324px
1920px
By no means would we include all of these (I hope that is obvious). We should absolutely let the content decide where we need a breakpoint. Also, we should follow the lead of the U.S. Web Standard breakpoints and not make ours device-specific because it is foolhardy to try and keep up with the plethora of devices. Here is an article that makes that point eloquently:
https://responsivedesign.is/articles/why-you-dont-need-device-specific-breakpoints
That said, I've found in past design systems the need to have minor breakpoints:
http://bradfrost.com/blog/post/7-habits-of-highly-effective-media-queries/#major-minor
The sizes greater than 800 come from here (give it time to load):
http://gs.statcounter.com/#console-resolution-na-monthly-201410-201509
My recommendation is the following:
480px
600px
1024px
1200px
1920px
My rationale is that the space between 600px and 1200px is too vast and includes an orientation change (portrait to landscape). We could choose to detect this with an orientation media query but I'd prefer the 1024px breakpoint. Also, treating everything above 1200px as a large desktop viewport misses an opportunity to take advantage of the increased space. Of course, it also means we have to generate designs for that space. Given that it has become such a common size (see link above) that strikes me as a worthwhile endeavor.