Phil suggested on IRC a LOCAL_VARS option that has the effect of making
all newly-declared variables local; e.g.,

% unset x y
% () { setopt localvars; x=42; typeset -g y=43 }
% echo $+x $+y
0 1
% 

I'm attaching a proof of concept patch (work in progress; see top of the
attachment for known issues), but WDYT of the the general concept?