Putting aside the prefix issue for the moment, my main request is to document which KM Variables must persist (not be deleted, like by the user when cleaning up his/her variable list).
Now, about the prefix. It does not matter so much what the prefix is, as it does to use a prefix, and hopefully in all macros. While I'd prefer there be one prefix we could all use, it is not required. Each macro developer could even have a prefix he/she uses. I like the "DND__" because it really jumps out at me when I view my KM variable list, but is hidden from the user in Prompt for Input. But that's just me. Use whatever you like.
I plan to mod my [KM] DELETE All Variables Except Those on Keep List macro to support as many prefixes as you like to NOT delete.