She already IS the greatest Earthbender ever... Lets take a look at her accomplishents, shall we Isabel? .
She created Metal-Bending, and is genuinely the first person to do it, making her a legendary master, and the creator of the most special and unique Earthbending we will ever lay our eyes on.
She single-handedly owned the Dai Li (These are the direct decendants from the Earthbending warriors who were trained by AVATAR KYOSHI).
She has defeated men 2-4 times her age with pure ease, and could probably defeat Bumi (Who is 100 years older than her). She has seriously proved experience doesn't mean anything when you are against her.
She is capable of bending what seems to be all kinds and types of Metal, and even imitated Iron Man's wardrobe in the finale, beating the living hell out of COMET-empowered Firebenders.
She is the human eqivalent to Badgermoles.
All that she has left to fully, FULLY, master, is Metal-Bending.
She is extremely young for a master, and will only become more of a legend with age...
She is the most unique bender in the whole damn show.
All this, and SHE IS A 12 YEAR OLD BLIND GIRL.
Excluding Avatar's (Who I personally don't think are as great of Earthbenders as she is), she is the greatest Earthbender we have ever seen, probably the greatest Earthbender we ever will see.