Well, I can think of a lot of pet names/affectionate nicknames for
Leah (LeeLee,
Leela, Leahbelle...) but none that are suitable for everyday use, except for maybe
Lee.
If you want to make the name stand out, maybe pair it with a short middle name like
Rose or
Jane and then call her by the whole thing (or make it a double first name)?
Leah is short enough for that to work.