It would be nice to draw a non-rectangular shape and do something equivalent of "select cells through" - where the region you draw is back projected "infinitely" and anything in the projection is selected? (Equivalent to the "free select tool" or "lasso" tool in GIMP) I'm finding it very hard to select some regions of a 3d model only using rectangles!
ParaView 4.0 will support selecting arbitrary regions. (Changes are already in ParaView git repository).
As in Gimp, the user should be able to choose if the new selection is added to an existing one or if it should replace the existing one. (subtraction or intersection of the selections as well as an undo button would be appropriate too)
A circular "brush" tool would be great as well.