The most frequent request is to move the TclX signal handling commands into Tcl. tclx signal commands for core is here for that. TclX is a grab-bag of useful functionality, a proving ground for new/nice facilities. They should be handled independently, IMHO.
Another suggestion that's been made in the past is to create an extension that provides Tcl bindings for the most useful Posix functions.
But what are the most useful Posix functions? :)
DKF: Useful Posix stuff (to me) has been: all the signal stuff, fork/execl, id and times. YMMV. :^)