This is a very interesting rabbit hole to go down! Multics certainly had some interesting features and now you can run it even on a Raspberry Pi 2. If they get TCPIP running on it it will be even more accessible to people...
Multics has a C compiler, but using PL/I would probably result in fewer memory errors...
They are also looking into ARPANET and Chaosnet.
More info on the AS/400 (aka "iSeries aka i5 aka System i)[0] and Multics[1]