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Put the smarts in the battery, or an inline module like the wireless bluetooth unit.
The shifters are simple switches.
The derailleur is a simple servo.
It would be easy to upgrade from 10 to 12.
davidual-
Di2 isn't simple - there are only two wires joining everything in parallel, except for the sprint switches which are just dumb switches.
Everything else, except junction Bs, has firmware in it. Last night I stumbled across Recessim, which aims to be a central repository for reverse engineering info... Maybe I'll get around to joining up to see if there's anyone interested in jailbreaking Di2. It would be sweet if I could replace the battery with something I ripped out of an old phone and an Arduino, and folks could make whatever shifters and derailers work together on anything.
Come to think of it, it would be even better if there was a tweakable open-source firmware for my crap telly... Wonder if that could ever be a thing, like with phones?
Kimmo
Di2 at least puts a lot of smarts in the battery, so replacing the cells is a non-trivial dismantling and soldering exercise that not many people will bother with.