personally, I think that (fn (a b) (+ a b)) is more readable than (\(a b) (+ a b)), and readability matters much more than number of characters.
Also, the [:] form could save more characters, if it automatically applied the outer set of parens to the body form.
However, I don't think it's really that much of an improvement; fn works well enough unless you really like extra syntax.
What was it the original poster wanted, anyway? It sounded like something that was more readable than _1 etc. for the var names; thus my dredging of the old thread. If not, then obviously, it wouldn't be a good choice. Maybe the [:] form should be capable of only naming some of the args, and leaving the rest to the other naming convention? Then the  form can name the first n arguments by putting them before a :, and have the args after that referenced by $, $0, $1, $2, etc. or some better character set, if _ looks bad.