A crazy project list in no particular order:
A softmodem that runs over SIP
An 80286 protected mode operating system, possibly OS/2 or Xenix/286 or QNX compatible
Toolchains for the above
A lightweight hypervisor capable of hosting any/all of the above
A Linux version of Microsoft's PNRP service
A user mode networking stack -- ideally supporting more than one protocol, interfaced through an LD_PRELOAD option like tsocks, or through the tun/tap interface