2 songs off ATYCLB (Elevation, Walk On)
4 songs off HTDAAB (Vertigo plus 3 others)
2 newly recorded songs specifically for the collection
So that gives us a total of 8 songs.
We need around 14 to 16 songs to come up with a decent collection.
Therefore, U2 needs to make two more albums after HTDAAB and 2011, and each album must contain at least three hit songs.
Considering U2's age and exponential productivity decline, that is a tall order. It is doable but the odds are against it.
I also strongly believe that for the new collection to be sort of a hit for the much older U2 fans, they will need to re-record perhaps a favorite song from the 80's and another from the 90's just so the songs won't be so unfamiliar for the much older fans.
Cheers,
J