This is a WIP issue to track potential changes for the next major version.

• Remove propTypes from core
• Add optional package for React propTypes
• Remove conversion of unitless numbers into pixels (most CSS-in-JS libraries support this by default now)
• Setup monorepo with Yarn workspaces and Lerna
• Split style functions into separate packages Consider optional package for additional style functions #281
• Create separate variants package
• Create separate propTypes package
• Create utilities package with themeGet, etc
• Remove support for implicit breakpoints
• Use sizes key per the Theme Specification
• (Under consideration) remove fractional width support – if you make use of this feature, please speak up
• Move experimental css and/or JSX pragma packages into monorepo
• Move should-forward-prop package into monorepo

Notes

• For the monorepo setup, and all-in-one package should be created to make migration from v4 to v5 as easy as possible

cc @neoziro