I do apologise for relating this to Oasis.
I believe a Wonderwall is whatever you want them/it to be. A situation, a person, an object.
For Noel Gallacher it is an imaginary friend who comes and saves you from yourself.
For me, its somebody who has everything wrapped into one. Bestfriend, love, parent. And saves you from anything you shouldn't be in, but it's very complicated.
And all the roads we had to walk are winding
And all the lights that light the way are blinding
There are many things that I would like to say to you but I don't know how
I said maybe, you're gong to be the one that saves me
And afterall you're my wonderwall