Have you looked at the STM32Lxxx  series?  Or STM8Lxxx if you don't need
 the horsepower.

Small foot print (7x7mm or smaller). Lots of peripherals with full power
management.
Fast wake up from sleep (8usec) and tons of other good stuff.
Lots of DMA channels for I2Cs,SPIs, UARTs, A/D, D/A etc. DMA to RAM or
device to device.
Makes your I2C overhead per byte next to nil. Cheaper than equivalent AVR.

I've just discovered this part, so still going up the learning/spec curve,
but WOW.
uA/ Mhz only slightly worse than MSP430 series, but full 32 bit core and
loads
of RAM Flash and EEPROM. Runs lowest power out of RAM.

Robert
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