I know this is not a "Best that never was" vehicle but speaking of SPAA vehicles based on the M113, I always liked the Italian SIDAM 25.
However to stay on topic and also with the theme of SPAA, there's always the ZSU-37-2 nicknamed "Yenisei".
Developed at the same time as the ZSU-23-4 but determined to be better suited for defence of tank units and SPAAM units but never entered service despite being considered acceptable (and also desirable) for service.
Some good images on the following page although the text appears to be a bad machine translation: -
http://survincity.com/2010/12/zsu-37...he-shilka-one/
And there's this discussion on wiki that reveals more details about the design/development and failure to enter service: -
https://en.wikipedia.org/wiki/Talk:ZSU-37