I asked the question for its own sake, and hoping to learn some interesting notion (this is appropriate for a forum, I guess); I am surely not so hopeful about further shrinking the current busybox.
That said, I object about the main point of some answers I got, that of adherence to a standard (POSIX).
First, I am not so sure that standardization is always the most critical goal.
Efficiency, neat design, modularity, etc... can conflict with that.
Secondly, compatibility should be relegated to a dedicated extension. I mean, IF it is possible to get away, in naked TCB, with a smaller (or even just somehow better, like plan9's rc), though not compatible shell, maybe it could be done, and then a separate ash.tcz be supplied for those wanting compatibility.
Others could use whatever they like: zsh, rc, pksh, or whatever.
Third, I regard some features normally included in shells as ugly duplications. For example the integer arithmetics construct $(()) can and should be replaced by a call to awk or whatever. I don't know if removing this from a shell would violate POSIX; if it did, I would like POSIX even less than I do now.