There are a lot of Keys that can be used for shortcuts.
Some of them:
0 or 1 or2 or ... 9
A or B or C or ... Z
Most of F1, ... F12
Alt+___ ,(one of the above)
Ctrl+___
Shift+___
Ctrl+Alt+___
Ctrl+Shift+___
DownArrow,UpArrow...PageDown...,Delete

I have not tried every combination...

Keys not allowed: ??????