There's a lot of what you say that is already able to be done by PA's Q in the way it currently operates.
I just want to point out something though that you're getting wrong. Foobar's Q doesn't operate like Spotify's and certainly not like PA's. Foobar's Q operates like what they call Linked Lists in Computer Science.
In Foobar, if I were playing from an Album A (so song a,b,c,d,...) and I put in something from Album B (so song 1), it will play song 1 and then continue to song 2, then 3, then 4 and so on. It will NOT go back to playing from Album A. That implementation is modelled after a linked list.
In Spotify, the Q actually does operate like PA's. It does take you back to album A, after you're done with the escapades into Album B (that are in the Q).
The only thing that PA does that Spotify doesn't do is give you the options to shuffle what's currently in the Q, and perhaps some other functions as it relates to the Library in general. It just works the way Andre described.
All in all, the Q is a separate playlist that works independently from the main playlist wherever that playlist is got from (categories, artist, album, songs).
Another point of difference but is related to the Q operations is how the shuffling options in Foobar are different to those in PA.
In PA, you can play a category (Albums, Songs, Genre, Folder) in order and then play within each category in (order or shuffled). And then you can play a category in shuffle and then play items within that category in (order or shuffled). You can decide to just shuffle categories, and shuffle within each category as well.
That is some brilliant setup and it's the only player that I've seen that does this. Spotify doesn't do it. MusicB doesn't. Neutron does not. iTunes does not. Winamp does not. And neither does Foobar. And those are the ones I've used