SERVER-8008 and SERVER-991 introduce the clause $slice to the $push update mod. The clause now only supports negative slices (trimming from the back of the array). The full $slice semantics support trimming from the front and also skipping (moved to issue SERVER10988). We'd like to support all these features here.
In terms of $sort, we'd like to use query semantics for that clause. For example,
This allows slicing from the front or back of the array: