I just noticed another point where the expl3 naming rules doesn't say what 
to do: what if I have a variable that is also a function? In traditional 
imperative languages those may seem like exclusive categories, but in 
(La)TeX programming it is sometimes perfectly natural, and quite 
straightforward.

The typical example of this would be a callback function: something that a 
module calls at specified points, but which the user of that module is 
expected to define. Treating this as just a function loses the specification 
of the scope for that variable. Treating it as a variable of type tl loses 
the specification that it takes arguments. What I found natural is to use a 
name of the form

   \<SCOPE>_<MODULE>_<DESCRIPTION>:<ARG-SPEC>

e.g.

   \l_harmless_callback:nn

This could be interpreted both as adding a SCOPE part to function names (the 
vast majority of functions without a SCOPE are implicity considered to be c_ 
constant), or alternatively as proclaiming any :<ARG-SPEC> to be a valid 
_<TYPE> for a variable.

Lars Hellström