I don't have a complete solution for you, just an idea that might get you started.
It's easy enough to reuse a KM variable set by a previous run of the macro.
For the numbers, you can just add 1 each time.
How do you handle incrementing the letters?
One approach is to use ASCII character codes, which are numeric.
The character "a" is 97. You can go from there.
I'm not sure if KM has a ASCII code-to-char converter, but if not it is easily enough done in AppleScript.
Good luck.