We need a language that goes one better, like a cross between Python and Ruby that uses two spaces for closed ranges and three spaces for open.

How about using different symbols?

   |-1..12-|.to_a ## e.g. months in a year
    #=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

    /1..\13.to_a  ## e.g. hours from 1h (1AM) to 13h (1PM)
    #=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

It requires compiler support, but hey, you said a new language.

I couldn't tell the difference for quite a few seconds there.

I see children losing hours on range(1, 12) in Python, have you noticed error? Unsafe defaults introduced to reuse function as

    for i in range(len) # 0, 1, ..., len - 1