My suggestion isn't too farfetched. A similar idea is given in this thread.
Basically, I was thinking that, in the Manage Collections screen, Instead of just "1. Delete the collection" and "2. Close", there could be a third button that turns on bindings for the three buttons to "+Set", "+", and "-" to the beatmap that you chose. I say that this only work with one collection at a time to prevent people from having like 50 different bindings for quick-adding or quick-removing maps from collections. So, if you wanted to quick-add (using bindings) a few songs to a collection called "Collection A" and then you wanted to add some songs to "Collection B" (also using bindings), you would have to go back to the Manage Collections screen and set "Collection B" as the bound collection. I will try and photoshop some screenshots to show you guys what I mean.
Basically, I was thinking that, in the Manage Collections screen, Instead of just "1. Delete the collection" and "2. Close", there could be a third button that turns on bindings for the three buttons to "+Set", "+", and "-" to the beatmap that you chose. I say that this only work with one collection at a time to prevent people from having like 50 different bindings for quick-adding or quick-removing maps from collections. So, if you wanted to quick-add (using bindings) a few songs to a collection called "Collection A" and then you wanted to add some songs to "Collection B" (also using bindings), you would have to go back to the Manage Collections screen and set "Collection B" as the bound collection. I will try and photoshop some screenshots to show you guys what I mean.