I think Lady Elieen is Erza's mother, 

Red Hair,

that look in her eyes,

the fact that one of the Spriggan 12 looked at Erza and was like, 'no way, . . . Lady Elieen' . . . . and also Lady Elieen was originally from Ishgar, 

See?! Coincidence? I think not! I bet Erza and Lady Elieen will battle before they realize who they are, or Lady Elieen will immediately  know who Erza is and she will either be filled with joy at finding her child or try and kill her saying how she's fairy scum now, and then after Erza defeats her, Lady Elieen will end up crying and say, 'You've become so beautiful and powerful' and maybe 'I'm proud my daughter can carry on the (insert Erza's actual last name) family name'. Either way I'm excited, for a while it felt Hiro Mashima lost his groove and a few chapters just seemed to be prolonging the story line because he didn't know what to do. 

BTW I will be posting the dates at the top so you guys can know when I came up with the theory, it's kind of my proof that I thought of it before anything happened, you know/