If you don't like _1, _2, etc. we could implement the system discussed here: http://arclanguage.org/item?id=8617. Basically, bind the unbound symbols inside the brackets in alphabetical order. It would work with _, _1, _2 as usual, but also x, y. Generally when people do short functions of a few variables they pick them in alphabetical order, so it makes sense. I'm just not sure how to go about implementing it. Presumably it would have to search the body for symbols which fail the bound predicate. sort them, and use them as the argument list. Unfortunately, it sounds like a macro with run-time knowledge. I don't know if macros have information on whether a symbol is bound or not; I would presume not.
You could look at make-br-fn on Anarki; it crawls through the [] functions finding free variables of the form it wants (_\d+ or __). It might be possible to modify it to pick everything out, if we want to. The downside, of course, is that if you typo, say, string as strnig, the [] function will think it's a brand-new variable. And the other downside is that lexically speaking, '_10 < '_2, though we could always special-case numbers.
Ok, I'll look at make-br-fn. And I hadn't thought about _10 < _2. However, when is someone going to write a 10 arg function with no names? Sounds suspicious to me. I agree that strnig would cause problems. However, unless you are very lucky, calling the arg "strnig" in function position will cause as much of an error as calling strnig in a normal function call. Of course, to see how useful it really is, I'd have to try it. I think that in most cases where all you want is three vars a, b, & c, or x, y, and z, it should work fine. And look better than _1 _2 _3, with less typing.
