Slic3r have, rotate, scale, split and copy. The only thing missing is mirror/flip to make all basic editing complete.
My suggestion would be: Independend scale for X, Y and Z.
Then you could also mirror, for example with scales X=-1, Y=1, Z=1 you could mirror around the Y-axis...
For easy use it could be locked to identical values per default, if unlocked with checkbox or similar you could enable independend mode.
But that is not my own idea:
If you use Repetier Host, the plater of Slic3r is deactivated and the Repetier Host provides in its GUI exactly the mentioned feature:
Flip in XYZ was implemented in e9910a2