A RIOT Hackathon will be held at Paris from 22 to 25 May and I think this is the perfect timing to handle the I2C refactoring introduced in .
the 22 May, an embargo will be created regarding the I2C API, all I2C drivers and all I2C CPUs peripherals. During this time, all PR’s related to I2C will be postponed/blocked.
A dedicated branch will also be create at the same time and an issue is already create for tracking the remodeling  and will be update soon . I hope this embargo will last 1 or 2 weeks max. This time will be used to update the API for all drivers and all CPU periphs. This refactoring will also introduce some new features like 10 bits addressing. I guess it will be impossible to implement all the new features for all CPUs in such a short amount of time. This is why we will implement these features on several (but not all) CPUs as a proof-of-concept during the embargo for testing purpose. I hope all CPUs will support these features for the 2018.07 release.
Any help on this scope is welcome (rewrite drivers, reviews, tests on hardware) even if you cannot attempt the Hackathon.
 https://github.com/RIOT-OS/RIOT/pull/6576  https://github.com/RIOT-OS/RIOT/issues/6577