I doubt that it's Midna... I mean she shattered the mirror, remember? Unless some idiot actually took his/her time to reassemble the mirror I don't see midna or the Twili coming back... well maybe a member of the Twili may return, you know how Zelda is, lol they'll never say it's a Twili, but you can look at him/her and tell who it is...

Besides although there are similarities, there are also plenty of differences... Midna is an adult while the blue figure looks more like a young girl(or at least it takes on the form of a young girl... I mean her/his pose says it all... the buckled knees, her arms(or lack of arms) in the cloak as if she's trying to stay warm, her skinny frame... she/he almost looks like a nervous girl who is lost trying to find her way back home... This contrasts with Midna's confident, sassy personality...

Given that Link is supposed to be an adult and more mature than any other hero in the past, it looks like she may be a lost young girl that clung to him and is looking to him to lead her home...

That's just my opinion, I obviously have no proof...